You sound a lot like me. Except that I'm worse.
I was a "perfect" mom to my son until he was about 2. We had this amazing bond. Then I got pregnant, and when I am pregnant I get very sick, so as a SAHM (with no family nearby) I was at a loss for what to do with my son. I really needed help, but didn't seek any. So I just became angry (not saying this is like you're situation, I'm just explaining what "changed" me). I started yelling at him and spanking him more and more. He became scared of me. And that made me mad at myself, which came back to him. It was an awful year. In the last couple of months I've been able to realize what I "turned in to", and how completely disgusted with myself I am. My husband has no problems spanking on occasion (the whole "we were spanked, and we're fine and love our parents" argument that I can't figure out how to contradict). And I have a long line or anger and abuse in my blood (I believe in generational curses, but even if you don't there's definitely something horrible spanning generations in my family).
I feel like the bond I had with my son is completely gone. I'm also having a hard time adjusting to his growth - now he's a real person that walks, talks, argues, jumps off of things, tells me stories, wants to understand, etc. I am clueless. I just don't know how to respond to him. I've mostly stopped spanking. I feel like my anger is definitely being absorbed by him. But I still feel like... a parent who has no idea what they're doing. Shouldn't you know by 3? I also feel that once my daughter was born, I expected much more of my not yet 3 year old than was fair to him. It was because I was exhausted with the baby, and had no idea how to juggle both of their needs. So my sons needs got somewhat ignored. I have finally put some money into getting regular help with my daughter, so that I can be less overwhelmed.
Anyway, don't feel too bad mama. Sometimes I get so depressed about spanking that it makes everything worse. I have started some counseling, which I think may help. I'm also looking for lots of help and support locally. And when I'm alone with my son, I try to think before I act. It's a simple concept, but surprisingly effective, although you have to train yourself to do it.