It's a tightrope walk sometimes. It's gonna be impossible *not* to stress or even occasionally trigger him. It happens no matter how careful you try to be and how compassionate you are. That's just the nature of PTSD.
The trick is not to take on the blame. It's not your fault if he is stressed or triggered unless you're intentionally doing things to trigger or stress him. That's all on him, and it's his responsibility to deal with that.
About half the time my vet can be supportive, the other half of the time my stress will send him over the edge. It's dependent on the situation, how symptomatic he is, and what else is going on. If he is doing well I tend to share more, but I always share in small increments and cut a lot of emotion out. I know he wants to be supportive. I know he'd be insulted if he wasn't in the loop.
However there are times when I don't tell him stuff. Out of respect for our life together I never hide anything that'd effect him personally... but if I'm dealing with something personal that is stressing me out, sometimes his meltdown isn't worth it. I'll choose to self-soothe rather than doubling my own stress. Sounds sucky... but it is sucky sometimes.
Reactions can range anywhere from him being a shit and getting irritated with me "for stressing him out" or "getting myself into a mess", to isolation/depression, to a full blown triggered state. The stress can be dealt with by setting boundaries. If he isolates he can go isolate, at least he's not lashing out. If he's lashing out, I refuse to engage... "Don't act like a shit to me, I won't tolerate it. I don't act like a shit to you when you're stressed out, so I'd appreciate the same courtesy out of a grown adult." We have that routine down by now.
Triggered is harder. I had a cancer scare about a year into our relationship. I had to have several biopsies, procedures, and some outpatient surgery. My vet flipped the f*ck out the day I had my surgery. He said he was a killer. He destroyed everything he loved. If I had cancer it was because he was a killer and he deserved to lose everything in his life. He started talking suicide. So that's how I got to recuperate from my surgery. I couldn't sleep because I was afraid he'd off himself if I did, and he was the one who volunteered to help me out for a few days while I was immobile. He shut down, so I had no help, no one to bring me anything to drink or eat, no one to help me to the restroom, etc. All this while I was in pain and waiting on biopsy results. I know he probably feels terrible about how he acted, but we've never discussed it.
This is supporter life. My vet works very hard on his reactions. He takes accountability for himself, and he doesnt use his PTSD as an excuse for treating me bad. He can respect my boundaries the vast majority of the time, and if not, he'll respond to me enforcing them. He communicates and tries. He has made a lot of improvement, and I appreciate how hard he works and how much effort he puts into "us". However there are times when it all hits the fan. It sucks when it just happens to be when I need him. We're only human.