I think I fall into the latter category, so here's my murky model and answers: 1) On two separate occasions, I recall explicitly turning down friends who wanted to create podcasts or writing projects. I was fairly sure I'd enjoy it, had expectations of high quality from the projects, and would have done it under different circumstances (being less busy). My initial feeling was fear of missing out--fear that it would be as fun as I anticipated, and that my reward would be "Well, I did have a chance to do that cool thing everyone else admires!" Later, it was more concern that I'd underestimated my ability to take on the responsibility. It was easier to decline because in both cases, they knew me personally, and I had more trust that they would believe me about being unable to take on more responsibility
2) I don't always use this model, and it could use improvement, but I weigh along stress and time invested. Time: how much practical time do I expect this to take, how well will that adapt to my schedule. (For instance, will it work with being totally unavailable during finals and midterms?) Time can obviously correlate with stress--something that eats into time to finish classes and other projects will inevitably cause stress. But stress also includes how much background noise the responsibility will cause in my life. Will I need to constantly be on the look out for podcast topics? Will I need to moderate comments continuously? Will I need to present myself differently on social media that has previously been simply, well, social? If I need to shift around other projects, will anticipating telling people that make me too anxious?
3) Yes.
4) Yes, up until the last year or so. I changed from a series of events. -Friends and classmates regularly remarked on how busy I was, which made me realize that they would be more open to me saying "No, too busy!" -I started with practical training in my psychological services classes, which involves a lot of learning how to get other people to set their boundaries. It generalized. -I started traveling more regularly, which meant having the easy excuse of being out of town, which meant getting more used to the idea that people took 'no' much more positively than I'd been imagining.
Some people seem to be a bit too generous for their own good. I know a precious few people who are especially good at saying "no" when asked to take on new responsibilities that would put them over their limits. I love working with people like that because I can always trust them to tell me when it would be better for me to find someone else to do the thing. I expect this to be an extremely valuable skill it would probably be good for many of us to understand, learn, and be able to teach to people who really need it.