I've had several friends in this exact scenario, where they just don't love their spouse anymore. The spouse didn't do anything wrong, they just didn't love them. To be honest, I usually tell them to leave. Not because I think THEY (my friends who are unsatisfied) deserve better, but because I think their spouses and kids do. A few of them left (not because I said so, they just eventually did). They ALL regretted it. It's easy not to like what you have until you lose it.
One friend got his wife to take him back, but their relationship stinks because she knows he didn't really love her to begin with, then couldn't hack it on his own, but realized she was great after all, and now for the kids, they're trying to be a family. Imo, it's not going to last.
You are looking at this in a totally self centered way, which is human nature. YES, you need to be happy. But happiness always comes from giving, turning your interest outward to how you can be of service to others by being a good mother, wife, friend. If you are feeling like you are forced into sex and you're so unhappy and it will never change, that's a FAR cry from trying. YOu should do a trial and say, "I'm going to fake it to make it. I'm going to enjoy sex with this person, initiate, etc for 3 months and see how I feel. I'm going to ask how He's doing, help him with HIS stuff, appreciate him, do stuff for him and forget MYSELF for just 3 months. Then, if I'm still miserable, I'm going to shove off" (If you can afford it).
If you can't be happy, and you knew all along you didn't love him, you need to give him a shot at a new wife who loves and cherishes him. He doesn't deserve this, you're right!
Also, I've been in a few past relationships with self centered people who I could tell felt a little "put out" to be in the relationship for whatever reason. I always cut those types loose with glee. Puleeeze, like I really NEED them so much? They always regretted it immediately, because they realized they were just looking at things wrong. And selfishly is always wrong. And immature. They were never trapped, and I didn't do anything to them. Actually, this was in my 20s, and they were just young. And then when they were all eager to try to make it work, it was too late, because I'm not into selfish immature people who feel "trapped" once they get something.
I'm not trying to be harsh, but think long and hard if you really want to get out into the world and find someone better than him, because if you put him through the hell of divorce, he's not going to take you back most likely. But if you truly don't love him, it's probably the best thing to do. If you're not sure, try REALLY REALLY hard for a few months to think of how to be good to him. The rewards can be great when you give instead of focus on what you're not getting.
Meanwhile, as an aside, I commend you for not cheating while having this dissatisfaction. I think you're a good person and will do the right thing here. You don't have to stay.