I've never been through this myself, but from what my friends have shared, one of the hard things is spending so much time by yourself, especially being alone with your thoughts. I would recommend filling time with things that feel personally productive and will help build you up. For instance, I read a ton because I love learning things - my current favorite topics are personal finance, land-lording, venture capital, domestic violence, education, parenting, business. You have your own list of topics that are interesting - google 'top books about X' or browse through Amazon, or ask friends for recommendations, and get those books from the library. Reading books during some of your alone time is wonderful. If you drink tea or coffee, perhaps make said beverage and settle in on the couch in comfy clothes. I absolutely look curling up with a book (or these forums) and a cup of chai tea with milk. (Or do this outside in a park, also lovely.)
Another thing I love to do is listen to podcasts - this way when I walk to school, I'm being productive and learning while walking. Or I'll put on a podcast and talk a long, hour+ walk around my city. (I also do the same while talking to friends on the phone.) Again, you'll have a different set of interests than me, but the podcasts I listen to (and recommend) are: Mad FIentist, Ric Edelman, Intelligence^2, Ted Radio Hour, Snap Judgment, RadioLab, 99% Invisible, On Point, Radical Personal Finance, The Moth, The Longest Shortest Time, Wait Wait Don't Tell Me, Dear Sugar, Invisibilia (not in season), Serial (not in season), Death Sex + Money, Mom and Dad Are Fighting, Savage Lovecast, This American Life, Planet Money, and Freakonomics.
What makes me feel most happy is when I have a community around me. Can you think of any activity or interest-based group hangout you can set up? Ie, if your friends want to read more, set up a book club that meets monthly where everyone rotates in hosting, and then you'll have this group activity to look forward to. Or an ultimate frisbee game every other Wednesday after work. Or a regular coffee date. Or start hosting friends for dinner.
take this with a grain of salt, my only long term relationship has been with my husband. I was only briefly involved with a handful of others before meeting him at 19.
I'm not the only person in the modern world who has had this experience? That is shocking
You're definitely not the only person - I'm in exactly the same position.