I've been through this too. Yes, you DO COUNT. Sometimes you have to be your own best friend and stand up for yourself, because no one else will. I find that actions speak louder than words. Hang in there, and do what is best for you.

Man do I feel this!  I remember many days of stepping in and being the parent becuause nobody else would.  I remember caring for my step children and loving them as my own.  I remember giving up things I really wanted to pay for thier sports that BM refused to.  I remember stressing worrying and doing my best to help them succeed, but I'm not really the parent.  I didn't give birth to them....Dang, it's not fair or right.  You deserve equal space in the good parts of the lives of these children as well as the bad/hard parts.  You don't get this because the birth parent(s) are insecure about their own place in the childs life.  This bleeds on to the children and thus we the steps are considered less...you aren't less and hopefully at some point these children will grow up and get it.  Until then disengaging can feel freeing even though you will likely be judged by those children for this as well.  Just know people like me (teachers, counselors, friends parents), we see you and know how important you are.  We see you and you are "real" and you're doing a good job.

With my Dad. My Dad who is my STEP dad. Ive been telling him how it is for me. Its hard to describe in just one sentence or paragraph. Or even mulitple paragraphs.

My Skids:
SD23 Feral Forger. Hated me as a teen, was a difficult personf for her parents to deal with. Her younger sister despises her. We tried, or rather I tried to build a bond when she was older (a few years ago) and that failed. She accuses me of taking her father away from her. Shes also a nasty pig who steals.

SD15/16 Backstabber/Munchkin. We were really good friends from 8.5-11 ish. I would spend afternoons at the park or the beach, or museums just her and I. I advocated for her, and we would have really great talks. Now of course she is super enmeshed with Toxic Troll her mother, and its hard to tell where one begins and the other ends. Pluas shes at that age of individuation. Plus shes been lying and manipulating. Every time I have stuck my neck out for her, my head gets chopped off. Im not to be considered. Ive always done something REALLY nice for her birthdays, things like strawberry picking with her friends culminating in chocolate fondue with sprinkles. A first trip to a fancy hair salon just because. Mani pedi just for fun. Lunches, live music. 

last year she didnt even say happy birthday. When something happens or Im sick there is absoulutel no "how are you doing Clove". 

Step relationship is very very non-reciprocal. We can give and give and do and help, and its all one way. I didnt do any of this to get something back, I am noticing that nothing is flowing my direction at all, everything is reserved for friends and PARENTS. The dog gets more sympathy than I do.

So, on to new directions and new flavors of disengagement. Now Im doing for me, and practicing the art of letting go. Which goes completely against my tenacious nature.

Sounds like you are getting a good handle on the emotional  gymnastics that is necessary to be disengaged.

I really don't agree with this.  

As a stepparent, you may not have an obligation to support or raise your Skids.. but all members of a household should be respectful to one another.  But, just by virtue of marrying someone's parent.. that also doesn't give you carte blanche to ride herd on your skids.. pretend you are their mom.. demand their love.. their affection.. or even a big place in THEIR lives.. you may be "dad's wife".. and that is OK.. as long as the household operates respectfully towards you.. and your partner PARENTS his children and expects good behavior.

You DO have a say in your household.. but so does your partner.. if the relationship is good? you are able to balance priorities.. needs.. wants of all the members of the household. and note.. I don't necesssarily say family.. because.. sometimes.. it's hard for skids to really view a sp as "family".. they didn't choose you..they didn't get a say.. and it's different than "you don't get to pick your parents." it just is and again.. being Mom's husband.. or "dad's wife" is a fine relationship as long as common courtesy.. reasonable boundaries and respect as a joint head of the household is given and supported by your spouse.  That may mean you don't get a say in their academic progress.. but you would get a say if they are destroying the walls of your home. etc...

I think a lot of Stepparents feel their POV regarding issues that don't have a direct impact on them or their assets should be just as valid as the bio parent's... but I think the bio parent has the ultimate decision over how those issues are decided.. a good partner will listen to your input.. but will ultimately make THE decision for their child.. as long as they understand the common understanding on expectations for the child after 18/HS.. and operate according to them.

Disengaging doesn't mean allowing skids to destroy your home.. steal from you.. it means that you stop caring if they take out the garbage.. whether they are on their phone too much.. whatever.. let your DH take out the garbage if that's what he wants to do.. and as long as they are meeting dad's school expectations (and he manages that).. whatever on the phones.. as long as it doesn't impact you personally.

Disengaging is freeing yourself from worry of outcomes because you are secure that your partner will deal with the fallout.

If you think something should be handle differently.  Then you get it's not your place.  You don't know, not a real parent. 
must be a real bio parent to really know.  But keep trying harder,  the harder you try the cool aid is easier to drink 

THIS!!!!!! - "Just shut your mouth and sit in the corner nicely while my kids walk all over you is the mind set of what a Step Parent should be.."

My SD17 doesn't walk all over me but the sitting in the corner is my DH's mindset. My opinions and ideas do not matter to DH. In fact he prefers I have nothing to say or do with SD17 but yet he expects me to be happy the 10 days a month she is here under our roof.

Her prom was last weekend. Her cousin (DH's niece) and her husband came to see her with their baby. She says "Can we get a pcture with the whole family? (meaning the 3 of them SD, her date and DH) and handed me the camera. And then niece and nephew stayed 3 hours!!!! They ruined my Friday night I could have enjoyed alone with DH. Just now  remembering how I felt last Friday s making me cry. And lucky me - Her graduation is next weekend. Another 2 days of feeling like a leeper!

How disregarded and unimportant an evil SM is. Like we are below the delivery man's third cousin.

It all comes down to how DH reacts to the total disregard of his wife. Does he back you up? Shut down their piss poor behavior? Or does he cower in the corner then kisses their royal asses?  Everyone is allowed an opinion, everyone has the right to not accept the SM. BUT dang if the SM disregards, dismisses, ignores the shitstorm skids SHE is the awful awful human.

I say give em what they think of you, espcially to the shitty adult skids.

Who said that you were required morally OR legally, to be a door mat?

Why do you keep inviting them to events?

I totally understand your willingness to keep the peace. THAT only goes so far.