OP, your other half and in laws sound like hard work & to put it simply, not very nice people. It sounds like they’ve completely broke you as a person and smashed all your independence and self worth.
Obviously none of us know your relationship. But from what you have told us, I would run a mile. Any decent man wouldn’t lay a finger on his partner. Any decent man wouldn’t allow his parents to control his future wife. Please don’t let any of them control you.
You are your own person. You have your own identity. Don’t let anybody speak for you, tell you what to do, tell you what to feel.
If you don’t want to go to the wedding. Don’t go. Ignore all the shit about “it’s rude to the bride and groom” .. “they will have paid for your own place” shit. They won’t care. They won’t notice. Loads of food gets wasted at wedding. Hundreds of pounds worth.
I would find it more rude if someone turned up to my wedding who didn’t want to be there. What is the point of having uncomfortable guests who would rather be anywhere else?
If your in laws think you’ll “ruin” the wedding by not attending, let them. Let them think whatever they want. Let them say whatever they want. It’s says more about them than you.
Now, tell your DP exactly how you feel. Tell him everything. Everything you’ve told us on this thread. Emphasise how his parents make you feel. Don’t hold back. If he’s worth marrying, then he’ll be supportive. If he’s not supportive, then he’s not worth marrying. At all.
If you go ahead and marry him. Then I wish you all the luck in the world and hope you can live a content life.
If you don’t marry him and leave him, then I promise this is not the end of the world. You will have the freedom to do whatever you want with your life away from the comments and behaviours of his parents.
Life is hard. Neither choices will be easy. But have a very good, long think about what you really want out of life. What do you desire your life to look like? You probably already know the answer deep down.