i don’t want to hurt anyone. What do I do?
You read up on 'parentification' & realise that it is not your job to reassure your dad, deal with his abandonment issues, or compensate him for his failed marriage.
You tell him - next time he has a go about your mother - to STFU & not to dare say a word against her to you.
You tell him - next time he whinges on about "losing you" or "shifts in the relationship" to stop being a self absorbed twat & try being a parent. If he then gets condescending again you could thrown in how creepy his words are coming across as - you are his daughter, not his g/f FFS.
You can spend some time looking up narcissistic traits & accept that your paternal GM is his Flying Monkey. That should help remove the sting from some of her nonsense. If it doesn't help enough - you could liberate yourself by telling her to stop being a nasty bitch & you will not tolerate her harrassment.
In short, you need to stop worrying about hurting people who are causing you quite a lot of hurt, pressuring you, manipulating you (dad) & gaslighting you (paternal GM). If hearing the facts about their behaviour hurts them, they have a clear choice - stop the behaviours, or prepare to hear more hard facts.
i just feel like this is my fault,
It isn't.
as I don’t want to see him anymore
Of course you don't. You are not mean to feel this way.
but I feel like I’m making a big deal out or nothing
You're not. He's been a disgrace.
and I’m just hurting him unnecessarily.
You are not HURTING HIM. He is HURTING YOU. Removing yourself from a source of pain is sanity. Insisting that your own child constantly reassures you & using her as an emotional punchbag (his spite & lies about your mother) is insane. You are allowed to remove yourself from that insanity.
I really don’t like his intensity and how I have to reassure him
Neither do I, It creeped me the fuck out just reading about it.
I’m not going to leave him.
Again - you are not his g/f. You are not your mother. You do not owe this man - father or not - your constant fealty. Stop looking at this through his perspective. Young women who have 'normal' dads don't need to worry about those dad's abandonment issues. It is desperately unfair of your dad to have conditioned you to do this.
When you are ready to see him again, have a few stock phrases at the ready. Things like "dad this is getting more intense than I want to deal with" & "dad you are overwhelming me with your emotions".
If he refuses to hear that & uses your entirely reaonable request to dial it down as another chance to bang on at you about how you need to show him even more affection & reassurance, you would be well within your rights to say "dad, you are doing it again, You make every meeting all about yourself & your feelings & what you think I ought to be doing in order to facilitate them. This is unfair & beyond what I can tolerate so I need to leave, & I will speak to you when you've had a chance to reflect & calm down."
And do it. Literally get up, pick up your bag, & get the fuck out of there.
You need to show him that his manipulations cause you to withdraw, & that if he wants to spend time with you, he needs to stop being so selfish.
I'm so sorry he & your GM put you through that OP.
You are NOT a bad person for wanting to avoid him, reduce contact, or even stop altogether. He's clearly been awful forever if you couldn't bear to see him between the ages of 10 & 16. That's not your fault - it's his.