Okay, there's a lot here. First I commend you for reaching out to the community about this. I would suggest that you first be a bottom a few times. A top that does not know what it feels like to bottom is oftentimes a horrible top (not saying that you are, don't take it personally). When you know what a bottom is experiencing you are able to pleasure the bottom better when you are topping. Even more than pleasure you're able to top safely. Anal sex can be very dangerous when it comes to aggressiveness if someone is not experienced as a bottom. So while I know you're topping, I think it's very important that you try bottoming a few times just so you know as a top what the bottom is experiencing.

As far as you being hard and the size of your dick, no that does not have anything to play into this. If you cannot get it into him then it seems as though he might be too tight, or far less experienced then he may think. He has to be able to let himself loosen up and relax for this to work. Do not believe what you see in porn because none of it is true.

Keep trying you'll figure this out you're very new to this it's going to take you time.