When I try to figure out how to balance personal happiness with saving the world, I think a lot about burnout. If I make a major change - e.g. changing careers - what are the chances I'll burn out and become a lot less useful as a result?
I've never burned out, so I don't know where that edge is for me (plus I suspect the edge moves around depending on circumstances). I'm obviously biased on the topic: there's the temptation to tell myself "This will prevent burnout and make me more effective in the long run" every time I want to do something.
Some things people here have described as causing burnout:
Going through the motions of a religion you don't believe in
Training yourself to feel guilty whenever you relax
Pursuing altruism too exclusively (That post suggests Bostrom's parliamentary model as an antidote).
Do you have more examples? Have you burned out? Are there things that you think have kept you from burning out?
This might be a re-phrasing of some of the other comments, but I think you need to calibrate your approach to match your personality make-up. For instance I could easily spend hours reading, thinking, and writing about some socio-political issue, but the idea of joining a march or protest addressing the same issue sounds draining. Other people are the exact opposite of that. Maybe you like traveling, maybe you like telling stories, maybe you like statistics, maybe you like street-art. Any of these could be creatively leveraged to change the world.
If you link your goals to activities that you can't get enough of, then burn-out is less of a problem. If you decide to equate the worth of your contributions with your degree of success in , because it's the culturally accepted standard (or even because it's the most effective tactic, all other things being equal), then you could end up both failing AND blaming yourself for it.