Rosebud, I think your lack of wanting sex probably has far more to do with the pain which is involved in having sex with your guy. I mean, why would you have a desire to do something which hurts you? And his lack of understanding that sex is causing you pain and constantly trying it on would only further turn you off, yes? And if you're turned off, then you won't want sex at all.
I don't really want to go into discussing his dimensions on the internet, but have you spoken with your doctor? I guess you'll have to figure out if his size is do-able with the right amount of arousal - or if he's just so big that it's never going to be pleasurable for you.
Lastly, lack of desire can also be attributed to emotional issues in the relationship. How does the rest of the relationship look? Are you happy? Do you manage your disagreements nicely? Are you in sync when it comes to how you spend your spare time, your morals, ethics and ideals? Be honest with yourself.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.