Steptalk will help you.  If nothing else, you can vent here.  It's good.

Welcome! Wow, certain aspects of our stories are so similar, I feel like I could have written them myself. You will find a lot of support here from people in similar situations. 

In my case, BM and DH dated in their senior year of high school to freshman year of college. During their toxic relationship, BM had several suicide attempts, exhibited self harm behavior, and ran away from home. Her therapists told her family and DH that it was attention seeking. DH stayed with her under the pretense of "how do you leave someone when they are at their lowest." BM's behavior continued and it led DH to start to self harming, so DH broke up with her. A year later, DH ran into BM's parents and they told him how great and stable BM was and that they should get back together. DH and BM started dating, but DH wanted to take it slow to not get sucked back in. Well after a few months, BM said she wanted to marry DH and have a baby together since they had been through "so much together." DH said "No." BM didn't take no for an answer and manipulated the birth control, getting off the pill without telling DH and altering the condoms. A few months later, BM ended up texting DH to break up with him out of the blue and two weeks after that texted him telling him she was pregnant. DH was devastated. BM and DH agreed to put SS up for adoption. Well 1 week before SS' birth, BM texts DH to tell him she is keeping SS but doesn't want anything from DH. DH sees SS the day after birth and BM says nothing. Then BM was mad because DH would not sign the birth certificate without establishing paternity via DNA test. 

Soon after, BM petitioned the courts for child support. Paternity and CS was established, but no visitation or custody was. The state defaulted to BM having sole custody due to laws about unwed parents in our state. DH fought to change this and lost. He was in and out of court for a year and a half. BM withheld SS for the first two years of SS' life and BM and DH had no contact during this time. When visitation was finally ordered, BM demanded it be minimal since DH "had abandoned SS for 2 years and SS didn't even know DH." The judge agreed ordering a few hours per week. 

I met DH when SS was 10 months old. When SS was 3, BM really ramped up. She emailed DH claiming that SS had cancer, he didn't. She started her control tactics, began commenting on everything in DH's life and kept visitation as short as possible. BM kept taking DH to mediation every year and she bullied him with threats of keeping SS away unless he agreed. When SS was 3, DH saw SS every other Sunday from 9 am to 2 pm with DH doing all transportation (we lived 1.5 hours from BM at this time) and the opposite Sundays from 4 pm to 8 pm at BM's house with BM present. The most we had was when SS turned 5 and DH got every other Sunday from 9 am to 6 pm with DH doing all transportation. DH begged for overnights and BM refused, stating she wasn't comfortable with DH having SS or that it wasn't safe. 

This whole time, BM made it clear she didn't want DH back. In 2017, SS was 5 and BM informed DH that she was moving SS 6 hours away out of state because she met a woman and wanted to be with her. DH sought legal counsel who informed him that we had no say due to lack of custody and just to create an effective long distance schedule. BM offered DH one overnight every month and 3 one week periods in the summer with BM doing all transportation. DH accepted because he had never had overnights. This schedule only lasted a year and BM changed it saying that the one overnight per month was too much for SS. She then offered every other school break and two two week periods in the summer with DH and BM splitting transportation. This is when things ramped up. BM started making false allegations, emailing novels daily, etc. We couldn't enroll SS in childcare or get medical attention for SS. BM kept pushing for DH to visit SS at BM's out of state, wanting DH to commit to either 7 weekends or 3 weeks of out of state visitation, stopping all visitation in our state. BM's GF even started getting involved in the abuse, claiming she was SS' other parent and even forcing SS to call her "Mom". They went as far as telling SS that DH was just a donor and they were his parents. GF was listed as the other parent on all of SS' medical and academic forms, blockingnDH's ability to access information. SS would tell us about the PAS but he didn't realize what it was/meant. He would just parrot BM's beliefs. It was hell for 2 years so DH took her to court for joint legal to stop the abuse and give him some power and more visitation. DH was only asking to go from 4 weeks of summer visitation to 6 weeks and keeping every thing else the same. BM made sexual abuse allegations during this time and ramped up the fighting. We spent a year in court and $30k to have the judge change nothing and tell us "pay CS and let SS go." Court wrapped up in 2020. BM and DH now only have email contact about once or twice a month. DH does his twice weekly FaceTime with SS and we follow the CO for visitation. We have given up fighting anything else and basically just let BM do whatever. BM won't be satisfied until DH has no visitation with SS, which is why she is picking apart the summer bit by bit. 

Since court, SS has worsened. He displays entitlement, rude behavior and just a generally poor attitude. SS believes that BM and GF are his true parents and DH and I are just "other relatives." The dynamic is obviously different between households. DH and I have given up pursuing anything outside of what we already have with SS10. I gave birth to DD earlier this year and DH is really great with her. BM sees this and it makes her mad because DH isn't as close to SS, which is BM's own making. BM's most recent control tactic has been making up medical conditions for SS since DD's birth and trying to get us to pay ridiculously high medical bills for "treatment." I think this is her last straw to keep control over DH. BM recently got engaged to GF but you can tell based on her recent actions that she still isn't happy. 

Theres a lot going on here! Thanks for the introduction. Our stories are all very similiar the differences are nuanced. Good that you have a court order. Good that he is building a positive relationship with his daughter, in spite of the BM struggling against it. I advise you to read up on posts here and look into the issues specific to Blended Families. They are the toughest "model" to manage, from what Ive read. 

Many issues will come into play for you: parental alienation syndrome is probably the biggest threat right now from what youve mentioned. The power plays that BM is doing, the weaponizing of the child against your husband. Once he gets his time, thats what they typically like to do once the child is able to discern who is what. At this age, kiddo just wants mom and dad and doesnt know anything else. There are things you can do, and emotional mindsets that you can move into, that will help either forestall or deal with it, if it does happen. 

Some OTHER things to watch out for, is "Disney Dad Syndrom" whereby your husband will feel guilty because he only has SD parttime and he has his bio full time so they try to "make it up". And there is also something to watch out for that happens mainly with SD's and thats "mini wife syndrom". All things that stem from the father and all things that are dealable only through him.

Additionally, something that will need to be addressed is life insurance policies. One for SD as beneficiary and one for you and your kiddo...I dont know your financial sitch but this is something that always seems to come up!

So - hopefully this wasnt too much - but I am feelilng ebulient today.