Is this normal?

I can sometimes get 80-100% erect by thinking about sex and touching myself a little; and sometimes I don't need to touch myself to get erect. The good thing is that if I touch my penis just a little bit or leave it alone, I can keep an erection for minutes. I think the longest I've kept an erection was for about 20 minutes? The weird thing is though, if I try to masturbate or stroke my penis faster and harder then my erection will go down. I don't know why this is happening? Maybe it's because I'm thinking about keeping my penis hard too much? I get anxious to see if my penis will stay up sometimes so this might have something to do with it. But has anyone had similar experiences?
Btw: When I say I tried to masturbate it was literally for a couple of seconds and I would stop. I've only tried it a handful of times. I don't know if that counts as edging but I don't consider myself edging either.