Gay male here, needing advice about 'topping'. [NSFW], I guess.

Hey guys. So, I have a problem. I'm eighteen, and I feel more than ready for penetrative sex. I've done plenty of oral, like blowjobs, rimming, nipple sucking, pit licking, etc, and I'm quite good with my hands, like with wanking and fingering. I've done this with a relatively large number of people, around six or seven, and I don't consider myself a virgin because of this. But, no matter what I do, I can't seem to get my penis into my boyfriends. The first time I ever tried it, my boyfriend had used no lube, or any fingering prior, and so obviously that didn't work out. However, that did ruin my self-esteem a lot, seeing as how I didn't know anything about anal sex at fifteen. My second boyfriend and I got together, and we did a lot of practising. He's able to fit two, sometimes three fingers there, he had lube, he could use markers, a lot of things. But my penis still wouldn't fit in, with or without condoms. The major problem, I feel, is pushing my head into the hole. When I was younger, around the ages of ten to eleven, I didn't know what erections were, so I used to push my penis down whenever I got one. My boyfriend suggested that this may be the cause. I'm also circumcised, if that makes a difference.

EDIT: Words. So, any advice to a newbie who reallys wants to be able to have anal sex?