it's probably normal for people to have serious doubts about whether they would make a good parent. i wouldn't rule out the idea of fatherhood just yet, unless of course you actually know you'd be terrible at it, like you're a pedophile or something. but perhaps the first part of your post is related to the second? putting yourself down will get in the way of a great many things in life.
To be honest, I pretty much only put myself down on the internet. Around friends and family I'm usually talking about how awesome I am and how they should strive to be more like me. Especially with my family. I say I'd be a terrible parent simply because I don't have any want for kids, and I really dislike even taking care of myself. I do it because I have to, but I resent myself for it all the time. I don't want to end up resenting kids too. People usually tell me they think I'd make a good dad. But that's only because I'm good with kids. What am I supposed to do, be a dick to a little baby? Also, I only have to spend a tiny bit of time with these kids. If I had them all day I'd become an alcoholic. And I don't even like alcohol.
Also, people fail to realize I need a girl to make a baby with. And I'm too busy with work and being incredibly lazy to go out and meet a girl I might think is worth having kids with.