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Are you very young (< 35)? If so, then I recommend talking to older people around you and hearing about their life experiences. I've found that the "fear of missing out" as a youth is assuaged when talking to folks whose most amazing life experiences occurred when they were easily twice my current age (puts a lot of things in perspective).

Also try this experiment: on one occasion, actively pursue an opportunity "overheard with friends" - when that activity is complete, recount the entire experience and decide if there was truly something you could have lived without. On another occasion, actively turn down such an opportunity and pursue something in the meantime. If you can do this once, do it again, and you'll start to learn to balance the pursuit of such opportunities and likewise a restraint. As motivation: If you don't turn down an opportunity then you are missing out on this experiment!