My parents are insensitive jerks
 

So here's the thing with my parents:

I told 'em flat out I'm not going to Wyoming. Cue them griping about how I've already seen my boyfriend this year, and that I will have my meds with me and can stay in the basement at the sibling's house, and when I pointed out that I'd TRIED to explain to mom that the sibling and I are at different points of being ready to deal with my mental health issues, she got dismissive again. and dad said that me being mad about the sibling saying he didn't trust the USPS and THEN SENDING BIRTHDAY GIFTS AND CARDS was "The stupidest shit I've ever heard, and I've heard some stupid shit from you."

Keeping in mind that I have not felt any need to take my emergency anxiety medication since January, and that I have made a huge improvement from where I was this time last year... I think I'm allowed to be selfish and look out for my best interests.

You do deserve to look out for your own interests! Dealing with parents is a difficult thing. I'm here for you!

Sad thing is, Mom KNOWS I'm not just doing this for attention. I'm thinking I may need to drag her back in for a mediated discussion with the therapist. And maybe drag Dad in for one, too.

Man, that sucks! I wish the best for you, though I can't really do much else. That's got to be frustrating!

I'm going to try and write a letter to the sibling, explaining why I'm not going to be present... but I honestly think I need to drag mom in for another mediated discussion, and maybe get her set up with a few appointments for therapy, because deities know she needs it.

I definitely agree that your parents could both stand to join you for some mediated discussion(s) and some therapy. It isn't right of them to try to force you to do something that could and would affect your mental health, especially when you've been doing better.
I really do hope that they can come around and see/understand your reasoning for not going.

Ugh. So I have a bunch of fall decor stuff that I've been arranging to find an arrangement I like to make a decoration piece to gift to the boyfriend's parents, and Mom stops and asks if it's for the sibling and his wife, then gets irritated with me when I point out that no, it's not, and that I had told her who it was for already. Then she asked me if I definitely was not going to Wyoming, and when I affirmed that, she sighed and said "Alright." Fucking Passive-Aggressive bullshit.