When I had severe daily pain I often agreed to plans or said yes to things I actually knew I wouldn't be able to do, just because I didn't have the courage to say no or set boundaries. Years later I was confronted with this, and I realized that what I hadn't wanted to say out loud or truly accept was "I can't do this, I'm too sick".
While at other times, having a chronic diagnosis could be an excuse for not having to explain yourself when cancelling plans. When everybody knows that you have serious health problems, maybe it feels like saying "I can't come, I'm having a bad day" is nicer to the other person than saying "I don't want to come, I want to do other stuff." I've definetely done that alot too, it just seems less brutal...
As my health improved, I realized that the people close to me had grown used to my evasion and didn't expect that I would keep my appointments anymore, because they were used to me backing out too often. It took a long time to regain their trust and confidence.
Today I am learning to practice rigorous honesty, even if it's painful, and rather ask for time to consider before saying yes to something so that I don't make promises I can't keep later. My honesty = my integrity, and is vitally important for my own self-esteem, and to keep my close relationships strong and healthy.