Sorry for the late contribution, I rarely look at the forums and just noticed your thread on the main page and was interested in helping.
I am a versatile bloke now but did not start to bottom with any veracity until I was in my 30s, well into my homosex years, so I definitely know what it's like to go through that pain of being fucked over a long period of time. My first top was about 10 inches long and thick (I am decent at just under 8 but he made me look like a little boy) and it tore me apart, a bad choice probably but he was bearded and hairy (and married) and way too hot to say no too! It felt like I needed to run away for a shit every 2 minutes… However in the years since I have managed to train myself to take nearly any size cock (certainly anything average or above average is no trouble), and it is NOT about being 'loose' or a 'whore' or any of those pathetic labels that gay guys like to use. Guys with good muscle control can be fisted one minute and tighten up around your cock the next, so 'looseness' is not the issue here. It is ALL about training your hole to do the opposite of what it has been doing for the majority of your life... i.e. pushing things out. Your sphincter's instinct from birth is to clam up as soon as anything tries to get in. It is this response that causes a good deal of the pain you're experiencing and you can train yourself not to do this.
Here are a few suggestions, some easier than others, but they are ALL within your control because it is your hole and you can do what you want with it. Please remember that the bottom ALWAYS calls the shots! It is NOT about giving your man a hole he can fuck regardless of you as a person. If you have any partners like that, give them a quick blowy and show them the door because they will not make love to you the way you want and need it at the moment, i.e. in a careful, considerate manner. Even if having your manhole used by a top is hot to you (which it is for a lot of guys) I would still avoid it until you have trained your hole up with partners you can trust, and who are interested in you getting off on it as much as in their own pleasure.
I met a very straight guy once (a sprinter) whose hole was so tight you could run your thumb over it and not even feel an opening. He was tighter than a fish and always wanted to be fucked, which was actually painful for me as a top trying to force it in! But after a few years and various partners he can take any cock he likes now. Still a very fit muscular guy with a tight cunt that he can control and contract to milk any cock inside him.
1. 'Starter' dildos or butt plugs are pretty cheap these days. You can get some very slender prostate massagers too (battery operated) which should slide in you quite easily and scratch that itch with no pain at all. Otherwise any smaller dildo will allow you to train yourself up (but only do it when you're horny and want it bad - by all means fantasise about some stud wanting to penetrate you and you 'opening up' for him, this will help your sphincter to actually dilate). Ask your mate to use one on you for a while first - remember to kiss him or suck him, work his cock and/or tits for him while he's doing it, which shouldn't be that difficult 😉
2. Don't get fucked on your back. Try sitting on it, or doggy, or sideways (spooning). Missionary (you on your back) should be avoided because it will allow his dick to probe you in painful spots far more easily, and although it's hornier it is a more difficult position certainly to start with. In the other positions you have the control to pull off when you need to. If you are in pain you should find after 5-10 minutes that your endorphins have kicked in and numbed the spot for you, and if it's still sore when he tries to get in you again then it's time to give it a break. Don't continue if it hurts.
3. As a bottom hygiene and consideration for the top is very important but it won't help you in the sphincter-relaxation department. I would consider douching much more than 15 minutes before a fuck, about three hours if possible. This reduces the likelihood of stray water that's made its way up your gut from coming out while you're in the act which can affect the lube on the condom as well.
4. Try and get with guys who are versatile. A guy who has bottomed and still does regularly will generally be more considerate towards someone he wants to fuck.
5. Keep practising and let your partner know that it's in his best interests to go slow. The more it hurts you the shorter the fuck will be!