I empathize with you and like you, have had twinges of envy seeing what my employed peers are doing. Ultimately though,
I'm spending my 20s building a foundation to do what I love doing someday, which is medicine and taking care of patients. There is beauty and satisfaction to be found in the small, seemingly mundane occurrences... if you choose to see it. The journey is the joy. If you've spent your 20s in school, as I have done... well, you can't miss what you never had in the first place!
There is no way to happiness... happiness is the way.
Shut down your facebook because those people travelling and appearing to have all this fun that you're not having certainly don't have perfect lives. They might wake up one day and realize they've wasted their potential by living directionless
lives in their 20s as opposed to working towards a professional degree that will give them the opportunity for limitless advancement. Or maybe they won't. Who knows? Your 20s should be about you finding your own path, whether that involves exploring the world or staying in one place, working to better yourself and reach your highest potential in your chosen field. As an example of someone lamenting a missed opportunity, my mother regrets not going for her PhD. She's 67 now and she still brings it up from time to time.
Life doesn't end when you hit 35 or 40!
That probably doesn't help you to feel better about your situation, but I thought it was a perspective worth considering, if you haven't thought of framing your life choices that way.
P.S. I'm not a med student yet, so perhaps this mentality is totally naive. Just speaking from the experience of being in undergrad for so long. Maybe medical school sucks 100x more and makes you feel like you're completely missing out on everything joyful in life? Hoping that's not the case!