How to tactfully "decommit" from projects?

Better to withdraw explicitly rather than just disappear.

After you decide which are the projects that you can and must stick with, go to the leader(s) of the others - in person - and tell them you must withdraw. Tell them your "youthful enthusiasm" overcame your good sense and you are in danger of shortchanging everyone's expectations and that you value their work too much to get in the way at this time.

A good time to be a bit honest and humble.

But this sort of thing has to be done face to face. Email isn't going to be a good vehicle. If face to face is actually impossible it is much harder. In such cases you may want to offer, in email, say, to stay connected but to take a lesser role. That is the sort of thing that would likely come up in a personal meeting in any case.

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Note that you don't have to be a youth to have "youthful enthusiasm". See, for example: Paul McCartney age 73