Hi Smer, I admittedly find it a little difficult to answer your question as it seems you raise a few different ones. Is it correct that your main struggle right now is to decide where to move your career, as you have a lot of different options and don't know which one to take? Or is that just one example, and you're more interested in the very general issue of finding it difficult to make decisions in the first place?

If it's the former, then I believe a more in-depth (maybe coaching kind of) conversation could be more fruitful here than a forum post, as I'm not sure broad answers to your post's title will be very applicable to your concrete situation. Also you mention a lot of things which you're already trying, but I find it difficult to see them in context as you didn't provide any notes on which ones of those work for you, and in what ways they already contribute (or not) to the central problem.

Anyway, just to try to add my two cents to the "how to deal with FOMO"-question, which I can relate to rather well as I'm also a 30-ish web developer often struggling with making decisions:

• I personally have the impression I'll just have to get used to living with the feeling of "what if this decision I'm making is not the best one?" - especially for big decisions that feeling will be there no matter what, so I might as well take it for what it is (a matter of subjective experience of uncertainty, and not actual evidence of the decision being suboptimal)
• delays caused by postponed decisions usually come at a cost, so quick suboptimal decisions are often better than ideal decisions made (too) late
• I sometimes use the book Decisive (or rather summaries thereof) to aid my decision-making, although I have a feeling it's often only more about about raising my confidence in a decision than about actually finding the best one
• The author of Algorithms to Live By makes the point that real world problems are often too complex to properly solve, and that it makes sense to artificially relax these problems into easier ones so we can find suboptimal but still pretty good solutions. That's mostly with regards to travelling salesman kinds of problems, and may or may not apply to personal decisionmaking.
• Career considerations may be a category where premature decisions come at a high cost, so here it really makes sense to spend a lot of time thinking them through thoroughly. Which probably includes discussing them with others. I'm not sure if 80,000 hours still offers 1-on-1 career consultation, but if they do, that may be a good thing to try, and if they don't I'm sure there are other people from the EA community who'd be willing to help out as well.