I think my dads new relationship is gross
What about his new relationship do you find gross?
I've seen his girlfriend in public places and I found her disgusting. She has an awful laugh and I'm worried she might become my stepmother.
It can be hard to accept a father's new partner, but you need to do your best to be supportive. If their new partner isn't abusive towards you and you simply don't like them because of their mannerisms, then you need to try to suck it up and support your father. If you reject your fathers new partner, then you may be denying him happiness and causing a rift in your relationship.
I can't handle it. This is the worst.
I get that you're frustrated and it's never easy when a parent dates someone new, and that can be true even if you like the new person. I think you should try to be more accepting and get to know her more. There's probably a reason your dad likes her and she might be a person you can learn to truly appreciate.