I have one child who is five years old and I have felt like this.
The good news is, it is getting better. I am hopeful that it will get better for you too. Funnily enough one thing that I have found since he turned about 3 is that it is very difficult and intense to be a parent of an only child. With a small age gap, you might find that the benefit of this comes at this age, because they begin to depend mainly on each other for their play, companionship, etc rather than on you, which means that your relationship with them becomes able to be more on your terms than theirs.
I really identify with Billy's post - and although I'm concerned that saying this will be less helpful and rather make you feel more trapped, I feel like I have to say it - if you leave your children, it will affect them. No matter how good your partner is, no matter how crappy a mother you feel you are, unless you present an actual danger to them, they are not better off without you.
I went through a really long time where I had to really force myself not to do the minimum with DS - and a lot of the time I really did do the minimum whenever I could get away with it. I felt sick with it. I felt a profound sense of loss and regret. There was one night a couple of months ago where I put DS to bed and felt genuinely elated because for the first time in maybe a year or two, I had suddenly realised that I had an answer to the question "What do you love about your children?" Because for a long time I couldn't have told you. To be honest, that still makes me feel sick now - that for such a long time I had nothing concrete that I liked about my child. Maybe if you'd asked me to think about it I could have come up with something, but it really felt as though I loved him only by default, because I had to. If he'd been a pet, I might have rehomed him. If I had an option to and I knew that it wouldn't have harmed him emotionally, I might have done that.
But, I'm glad I didn't have that option, endlessly glad. And although I still have moments where I hate it I can truly say that I enjoy it now. Most of the time. I had DS young and it's frustrating to see other people my age having a totally different life to us.
I think that feeling let down, or lied to, or worrying that you've somehow failed or aren't a good mother because you don't "feel totally fulfilled" by having children is far more common than people think. Also a big part of it is just that it is a massive, MASSIVE headfuck, shock to the system, lifestyle change to have children and it doesn't matter what people tell you in words, words cannot convey how that actually feels. I don't know what your ideas were about having DC before you had them, but I always wanted them, thought I'd have loads, assumed I'd be a great mum, etc, and therefore I feel far more like I've failed, been cheated, I don't know? Maybe it's not anything to do with that. But I suppose it's not like I imagined it and I don't know why, and I suspect very much that it's me. I do have very high expectations of myself as a mother and it's easier in areas where I can either relax or easily achieve those expectations. I've also found it easier to try and redefine my thoughts about motherhood along the lines of thinking of it more as a relationship (as I suspect DH does) and less of a role or task that I can do well or badly at. Because when it's a task, it's never ending, and that makes it frustrating and mundane and boring. But when it's a relationship it's easier to be easier on yourself, it feels like a positive that it stretches on endlessly rather than a frightening prospect.