Search lpsg by the term "selffuck" and you will get some other threads about this.
I successfully got some of my cock inside my ass a few times, but haven't tried in a while because the experience was way better in my mind than it was in real life.
You're right about the angle thing for this reason: If your cock is super hard and won't bend, then how is it physically possible to get it inside your hole? I think the answer is that it's not possible, your cock has to be a little bit soft, enough to bend at least a little, to allow the head to get in to your hole. What that meant for me, then, was that I was in a state of not-full arousal - because if I got fully hard while inside myself my cock popped out.
Also the angle made it difficult, for me anyway, to do much once inside. I could move my half-hard cock back and forth a little bit inside my ass. So what? I certainly was never able to do my ultimate fantasy at the time, which was to cum inside myself.
The trick (which I learned here I think) about getting in initially is to kind of fold the penis a little bit and push with the area just behind the head...then the whole thing pops in, head and all. Kind of hard to describe. But trying to push in with the half-hard cock head doesn't seem to work.
My $0.02
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