I agree with Grandma TM - it's worth it to try to work through these problems first, but realize that if the other partners in this (your husband, child & stepkids) won't work with you, you may need to get out.
Maybe you could sit down and have a heart to heart with your husband and kids. Sit down and let them know where you're coming from. Don't be accusatory at all, but use "I" statements, and do take responsibility for how you are acting by saying something like, "I know you have all noticed how crabby/unpleasant, etc. I've been these last few years." then let them know that you are not happy, and that you're asking for their help---just as you have helped them. It takes work on everyone's part for a family to stay a family, and everyone in the family has jobs and can contribute to the family. Start small, by asking for their help with some things (like maybe a rotating schedule of doing dishes/laundry/minor cleaning).
Or, if that is too daunting, sit down and have a heart to heart with your husband. Before you do, do put together a list of what you do: your job as a mom/homemaker. It is a job, and if you can list all the things you do, and how much time it takes, and how much it would cost for someone else to do it, it may help to get a perspective. Then, let him know that you are unhappy, and ask him to help you with actions -- if he's a decent guy, he'll want to "fix" this for you, so try brainstorming things that might help. Again, don't be accusatory, but do let him know that you want/need his help and that you do love him.
Counseling may also be a really big help, since it sounds like you are really depressed and very stressed. See if you can get your husband to go, but if not, go for yourself.
Also, I think that you should find something that you're interested in and do something for yourself. It's amazing how taking just a little time for yourself, to do something fun or that you're interested in, can be hugely revitalizing. I just started taking a class through the rec center and even though it's a huge pain to schedule childcare each weekend (even for the hour of the class itself), it has been so worth it to do something for me. :) Start going to church or volunteering, if you're not sure where to start. Start developing outside interests and friends--this can help tremendously.
If you still have any small spark of love for your husband and your kids/stepkids, try to work with through this. But also know that if they aren't willing to work with you, then you need to get a lawyer and separate/divorce anyway. But at least then, you'll know you did what you could to try to work things out.
Lastly, if it comes to it, you have worked very hard over the years for your family, and for your husband, so you do deserve to get what you can legally and fairly. I'm not saying to take him to the cleaners if you decide to end the marriage. But I am saying that I've had friends who gotten divorced and their good nature (or their depression, or both) got them taken advantage of and they were basically left with nothing but the shirts on their backs, even though they deserved more. So, if you decided to divorce, get a lawyer and do it before you tell your husband that you're going to, because it sounds like right now, your husband is not feeling particularly compassionate or fair, and he really won't be if you tell him you have a lawyer, probably.