I just want to let you know you're not the only one going through much of the same situation. We have a blended family also, with a "mine, his, and ours" family structure. We separated once before for a while and then started going to counseling together. It really did help a lot, but now, 4 years later, I see us kind of going back to that relationship dynamic level.
One thing our counselor told us to do is to take a piece of paper and write down 10 things about the other person that you really like. This is something that you could do at home. It's really silly feeling, but it helps you to feel what you had once felt for each other, especially after you lose sight of what it was that attracted you to each other in the first place.
I've read some books and web sites about marriage and relationships and making it work. One thing I learned is that the way you talk to each other makes a huge difference. Taking a proactive tone of voice when discussing differences with each other helps; instead of accusing each other, say something like, You know, I really appreciate when you [help with supper, chores, picking kids up from after-school activities, whatever]. And I know I am not always the ideal wife and that we don't see eye-to-eye on many things. I was thinking that something we could work on together is [insert one of your marital stressors here].
Yeah it does sound really cornball and hokey! But you know how men are - if you come right out and say, I don't like it when you...., they are going to put those defenses up and be ready to battle.
It does sound a lot like you want your marriage to stay together. But if you really don't think it is going to work between you, then it may be best for both of you to separate. You know your husband better than any of us do, so if you think he would be willing to work at the marriage and put forth 100% effort to keeping it together, then make the same commitment yourself. Either way, you have to communicate. That was our biggest problem, we didn't communicate, and it is still so hard for us! But I've learned that 99% of the battle with that is how you talk to each other.
Please feel free to message me if you want someone to talk to. I'll keep you both in my prayers.