I'm an adult living with my parents so I've struggled with this a bit myself. Obviously, if you can avoid moving back in then great, but if, like me, you have no choice then I would suggest writing them a letter about how you feel or sitting down and talking with them if you're comfortable with that. Also, highly recommend if you're seeing a doctor or therapist who's helping you with your transition (or if you can find one) that you bring your parents along. They are probably way more likely to listen to another adult with authority than they are to you (their child).
As for seeming ok with things and then going back on their acceptance, I'm realizing with my own parents that a lot of this transition stuff doesn't seem real until they're forced to confront it. So if you've been living far away this probably all seemed like a far away dream to them or maybe even a phase they secretly hoped you would grow out of. But now that you're in their space again, they're having to confront what this might mean for them. If they're not the sort to kick you out it may just be a question of time and getting used to things and seeing that you are really set on this. That's why it's so important not to put your life on hold for them. Who knows if they will ever be ok with it? And, as much as it sucks to not be taken seriously, moving forward with something like T may actually force them to confront the reality of this in a way that pronouns sadly don't. If that's what you really want to do, that is.