He's probably not as big as that. Someone on LPSG issued a challenge with a prize of $10,000 to anyone who had a legitimate 10" dick and no one has claimed it yet. Still this doesn't help your problem.
You do not say where the pain is coming from, which is important.
The first issue is girth and your anus, the second issue is length and then him bashing against your internal limits. The third is how he's fucking you.
To the first issue. The thicker he is the more of a stretch will be demanded of your hole. Usually this isn't a problem, beyond needing you to just relax, and I would have thought you'd have had this covered by using toys that were at least as thick as he is. Nevertheless there are some caveats. Even if you are relaxed, the anus is made of very sensitive and relatively thin skin, it is prone to damage from pooping and then not healing properly after said damage occurs. The more you stretch your hole the more old injuries can cause a problem and will usually sting or cause a sharp pain in your hole. There isn't a lot you can do about this except gentle play with thicker and thicker toys to allow the damaged bits of skin to hopefully stretch and then be comfortable with his girth.
The next caveat is lube. I'm assuming the lube you're using works for you but here goes anyway. Most water based lubes contain parabens and glycerol. On the whole these are great, as most people do not react to them, but if you do react to them then they can cause a burning sensation on your hole. This is very unpleasant but the sensitivity varies a lot, from person to person, to the point where one person might not really feel any pain with a regular sized toy, but when a larger object is used and then thrusted, things start to hurt. Try going with a lube that is free from these additives, such as H20/Sassy, from Sliquid. The other alternative is to try a silicone lube, such as Gun Oil silicone lube. My own experience with silicone lubes is that they aren't quite as slick as water based ones and although things slide there's a little bit more friction. You do not want to use any lube with numbing agents in though. This might seem like a good idea but it's the pain that's letting you know something is wrong. You want it to hurt if you're going to damage yourself so you know to stop.
The second issue, with his length, is something that doesn't get enough attention in my opinion. All guides are focused on getting clean and relaxing the hole, but rarely cover taking the length comfortably. The average rectum is only around 6" deep and it isn't a nice straight passage to put your penis through. If you google anatomical images of the anus/rectum/sigmoid colon and look at them from the side and from the front you'll see how they bend and curve. The nice thing about the human body is that it has a reasonable amount of flexibility so you can rearrange things into a more direct route but, nevertheless, if his penis curves one direction and you curve the opposite inside he will poke and bash into things. This hurts. Some people like this pain, most do not. One advantage to all the monster dicks on TimTales is that they tend not to get 100% hard. You'd think this would be a bad thing but if the dick is a little less hard then it can bend where it needs to go. Also if it's less hard, when it does bash into something, it's nowhere near as solid, helping to cushion the blow. Maybe your boyfriend is the one who needs the beer, not you!
If this wasn't enough depth is further complicated by a bunch of extra obstacles. The first of these are known as the rectal valves. These are designed to block off portions of the rectum and naturally open up as the rectum fills. But after you've douched, and are empty, they will close off. You need to slowly open yourself up and gently tease them apart. If you bash into them with a hard dick it hurts and then aches afterwards, so this is not a good start. After the series of rectal valves you've got a major obstacle and that's the sphincter that goes from the rectum and into the sigmoid colon. This is, rather unglamorously, referred to a the 'second hole' and is usually tightly closed off. If you bash into this it's really unpleasant and bashing into it repeatedly is no fun at all. To open this up you need to take your boyfriends dick deep enough so that his cockhead is nestled next to the second hole and then gently rub his cockhead against it pushing gently until it opens up. When it does open up what's left of his length should easily slide in. For him to fuck you comfortably all of this needs to be wide open. If he should change pace and go from deep penetration to shallow thrusts, say he's just teasing your anus for a bit, he needs to watch out when going deep again because things can close off without warning.
I don't know how he's trying to fuck you now but it goes without saying that pounding you like a pneumatic drill might never happen depending on what is causing the pain. Once you're opened up slow deep strokes are necessary for you to gain confidence and to allow yourself to trust in your body so you can relax properly. If you are nervous about taking his dick then this makes it harder for all the bits inside to open up and increases the chance that they'll close off again.
If depth is the problem then you could always get something like this.
Silicone Penis Cock Ring Thursting Buffer Bumper Donut Cushion Stopper Ring | eBay
He can put it on to prevent accidental deep penetration.
