I'm not sure if they do Botox in the UK for that reason or if it's only private clinics that do that. But for vaginismus, when muscles cramp up unintentionally making even finger penetration hurt, it numbs those muscles and makes it harder for them to cramp. So it's not that you lose feeling, it's just that it's more difficult to flex them.
If it's that part that's hurting it might just be that he is going too fast and possibly a bit too big. It was similar with my boyfriend to begin with, just because he's a bit above average. I have masochistic tendencies tho so I didn't really mind it. But it would feel like he might split me open and would take me a moment to relax. And then leave me sore for a couple of days. Having some finger stretching and going at your own pace helps. Now I barely feel the pain anymore.
And it was similar with doing it doggy style too. To begin with he would occasionally thrust too deep in that position but now I'm totally fine with it and actually enjoy how deep he goes.
It may be worth seeing if you can get him to go slow and slowly build up the speed. That may help if he generally likes it fast. Trying some of the things you tried before may also work. When I first tried being on top, I didn't really enjoy it that much and the positioning was awkward. Now it's one of my favourite positions.
And both the vagina and the guys penis may have different curves to it so for some girls being backwards on top is more comfortable than being forwards. So trying things slightly differently can help. And when you're on top, you don't have to go all the way in on the first go. You can have it go in a little bit. Lift up, give yourself a moment and go slightly deeper on the next go. As long as your partner stays still and gives you the time.
If you want to, you can pm me if that's more comfortable to you because I don't mind these kind of conversations and it's always nice to talk to someone about it