Hi
lighthouse9 Just providing some support for your self care.

3. Drink more water and eat more vegetables.
Great basics you have here. I noticed water actually cheers me up on some days when things get tough. Not hydrating seems to cause the opposite.

5. Find humor and whimsy in whatever way I can. I bought a ridiculous shower curtain for my new place (it's of a cat riding a narwhal) despite protests from my mom that it wasn't very "adult."
I'm a big fan of this. I have quite a lot of fun or silly work stuff. Just because you like cats riding other animals has no connection to how capable you might be in other areas of your life.

6. Get back to working out.
I am so envious. It is so hard to maintain this when there's so much other stuff going on. Good luck!
... .mood changes linked to seasonal changes? Do you need to double down on self-care?
For everyone, there are up days and down days--and yes I will increase self-care if it's not a going well. Guys have moody days too. A new day will begin tomorrow.

4. Confront a fear or two. Since the break up, my anxiety has been claiming situations as dangerous or uncomfortable and I've been avoiding them. ... .
I support your fear confrontation. I'd like to share a tip the about situational hypervigilance. For me, what was driving it was heightened self-consciousness of "prohibited behaviours" by my ex. After that, I wanted to have nothing to do with anything like her. So the energy kinda moved to vigilance for "BPD" signals. That's fine but I found it easier to just focus on the fun and the moment and make that happen--then see what comes of it. If you've already figured out what signals are important to you and you're able to not ignore the stuff you don't want when things are going good--then I think it will benefit you to focus on the fun instead. Yes, there's a lot of people with PDs out there, but if 6% meet BPD diagnosis, that leaves 94% that haven't. That's heaps of people that probably won't trip your "has BPD" wires. Even if you want to extend the stat to 10%, that still leaves a good 90% of people.
When a relationship progresses, the BPD signals will probably manifest more obviously to you as you won't be so "spent" on things that don't matter, if they manifest at all. I hope this helps you get more freedom and peace.

Enjoy your peace lighthouse9.