Here are two stories that might be relevant.

I know of someone who anticipated burnout from teaching in an urban school district. He gave himself eight years in that line of work. After those eight years, he planned to move on. Which he did. He is at peace now, both with his current work and also with the knowledge of having made a significant contribution.

I myself used to listen on a telephone hotline, coincidentally (maybe not so coincidentally) also for eight years. We handled suicide calls but the large majority of the calls were from people who were depressed and/or lonely and/or shut-ins. It was really valuable and important work and I enjoyed it and I never lost interest in the suicide calls but I did eventually, after eight years, start to lose interest in the regular calls. I knew then that I had to move on.

Well in both of my stories the people involved did not bounce back into what they were doing. We both though did find other philanthropic things to do, that better fit us at our current stages of life. Perhaps this would be a possibility for you?

This is not selfish. Taking care of ennie is also important. These inner rumblings might be saying that it is time for you to change, to use other avenues in your effort to help others, or perhaps to focus on yourself for a time.