I find that an important component in my relationship with my partner and navigating mental health issues is ensuring that they aren't my sole support system. Making sure I have other supports (friends, therapist(s), skills) within my network so my partner doesn't take on the role of 'therapist'.
From what I can tell, a lot of stress (at least with us) can happen when one half of the partnership feels like they're the only 'safety net' or catchall for the feelings or symptoms.
And as predictable as it might sound, I think that communication is key. If I need to talk to my partner about something, I am able to intro it as, "--I'm having serious issues with X & Y right now. Do you have the space to listen?" (Granted this is when I'm able to communicate + progress in a more logical manner; there are times when things are perhaps too haywire for this to proceed so simply)
The most difficult thing for me is to not take it personally when they don't have the bandwidth as we all have our struggles.
This link has some general ideas of maintaining boundaries while being supportive that you might have already come across. Also, depending on where you live, maybe there's a support group for spouses/relatives with X. I think those safe spaces are really healing because (from what I've been told) loved ones often feel like they're betraying their spouse/relative that's having a rough time by venting but amongst others who understand, it can be easier.