I wrote basically this in another comment, they had a hard time recognising their emotional state and held until bursting out. This was way before they were diagnosed though, after a lot of therapy they are much better at identifying their state, or simply they try to keep in touch a lot more so that I can be there for them before they burn out.

As the neurotypical person in the relationship, my advice is to try to keep in tough more regularly so that your SO can detect if you are halfway through burning out so that they can help you before you become completely unavailable.

My SO also has generalised anxiety and ADHD, so I usually tell them that when they keep burning out for weeks it's really painful for me because I feel very left out, which resonates a lot with them. I guess that this helps them to do the effort of keeping me up to date so that I can let them vent, hug them, ask which kind of food they would like to uplift their spirits... and all that stuff before they burn out.