I'm going to turn down my long-time boyfriend's proposal
Are you joking? Why would you turn him down?
I can't stand him anymore, his family is awful
Being rejected after proposing can be difficult. Try to let them down gently be firmly so that its conclusive. This way they hopefully understand and don't keep pursuing you.
Maybe he'll change.
If you think he can change then you should stay with him. But if you don't think it's gonna work out then it's okay to turn him down gently.