It's a red flag when a spouse doesn't want to put the effort in understanding and work something out. The fact that he dismisses it and could care less how it affects you lets me know that you are in for many years of living like this, until you take action.

I am so frustrated and just sick of being crushed by having hopes that maybe it won't always be so hard.

^^^^^^It's always going to be this way until he decides to fix it. He has to care and want to make the change. Until that time comes.....you are are in for one heck of a ride.

I can relate with you in a lot of ways - Dad doesn't understand or empathize, and won't take the initiative to make it "better."  It's always what you could have done, or should have done, or why it's YOU and not him, not his SS, or not anyone else.  It sucks, it makes you feel alone and that no one understands all of your complex feelings (being pregnant and navigating step life is EVEN harder- I had two induced pregnancies because of stress). 

It sucks not having the father of your child experience all of those "firsts" (referring to your 1.5 year old) that you did, because he did it already.  That it feels like two families- one with him, your children and your SS; and the other just you, him and your children.  However, no matter if its intentional or not, your SS 's presence really makes things, well, crappy.  Your best day with your husband will turn into your worst as soon as SS walks in the door- it's almost like everyones' mood shifts with him there, and the energy is different.  There. is. a. stranger. in. your. house!! 

It would just be so much better- honestly- without him.  Not worrying about schedules, or hurt feelings, or children who feel like they are deprived of their father and therefore act out emotionally, or exes interfering, feeling like you are competing for your husbands' affection with a child, making compromises that affect you and your childrens' lives, etc. etc. It's all a messy, difficult complex situation.  And deep down, you don't really love SS.  You'll help take care of him, you understand he's a child, and you'll make sure he is fine, but, that doesn't mean your heart accepts him the way your heart accepts your own babies. 

All of this is perfectly fine to think or feel- there is no right or wrong way to address step life (unless of course someone is being harmed), but most of the time step parents are always going to wrestle with those negative, ugly feelings that we have to keep stifled a lot of the time.  I don't have to grab a megaphone and yell that I don't love my step children for my partner to realize that, but I try not to make it such an apparent thing all the time.  Essentially, I try to just deal with it.  And he does too.  Which, is one of the reasons why we are in the midst of trying to end our relationship, but neither one of us has really "pulled the trigger" so to speak.  

My advice for you, in the moment, is to just focus on your upcoming birth of your baby.  That child is going to need your stability, your love, and your presence in its life.  I was depressed, stressed and unsure during both of my pregnancies because my partner and my step children made me MISERABLE.  And you know what?  I missed out on a lot of the joys of just being pregnant because I was always dwelling on what could have, should have and would have been.  I know that is hard to do, but don't waste any more time worrying about things you can't change (against, this is hard, and I can't even do this all the time- but I wish I could).  I have also found that it's easier for me to disengage in some ways because I am so wrapped up in my two childrens' care.  They keep me busy, and yours will too.  Find happiness in the little things they bring YOU and that you bring THEM. 

We grow everyday as people, and I think for some of us, we slowly grow stronger until one day we find that strength to do whatever it is we have been wishing we could for so long.  That may happen for you one day, and you realize you're over it.  I am not saying your relationship with your husband is or is not worth fighting for.  I think over time, collecting all of these good and bad experiences, evaluating his role in your and your children's lives, the role of your SS, etc., you'll just know what you're supposed to do. 

I am just one person, but my experience (and opinion) is that if there's already some cracks in the step-life foundation, they will most likely continue.  These cracks aren't just the "fault" of one person, but from the dynamic as a whole.  Even if it seems like it's just one person- it almost always isn't.  That doesn't mean you- but, from your example, your husband and your parenting styles are different, SS brings a lot of stress and tension to your relationship, time isn't equally balanced, you are already emotionally distressed, etc. 

This sounds so depressing, doesn't it?  I am not trying to do that, I am just trying to say that depending on your idea of "happiness" within your relationship, it probably won't get much better than it is, especially with another child coming.  I know my partner became even more distant when our second child came along, because he felt even more guilty about not giving time to his first two (he's gone all the time now with them, which just leaves me and my children alone all the time, which doesn't cause him as much guilt for whatever reason).  There is no conceivable way to really understand the complexities of step-life without actually living it, and for some of us, we realize once we are in it, some deeper than others, that it might not be for us. 

It's just up to us, at that point, to decide how we are going to "fix" it.  Whether that's learning coping strategies, disengaging, or even ending relationships, we have to realize that we matter just as much as anyone else, including our children.  WE are someones children!  Our parent(s) want us to be happy too!  When our children are adults, are we going to be concerned when they aren't happy?  Of course we are.  We are going to do everything in our power to make sure they are, not tell them to go sit on somebody else's back burner because they are adults and their happiness doesn't matter anymore. 

It's hard for us to remember that sometimes.  For the time being, maybe bide your time.  You have a lot going on in your life.  Your children are going to grow - your youngest will need a whole lot of attention once they are born.  Your SS will grow too; I can't tell you that it will get better as time goes on, as I've heard that the teenage years are a whole other breed.  But you'll definitely be a first hand witness as to how the dynamic of the family changes with a new little one.  Keep yourself immersed with your children and your motherly responsibilities and slowly figure the rest out.