When I was 21, I moved out of my parents house and moved from Orlando to Dallas for a girl I met on the internet. We were together for about six months and then she broke up with me. Additionally, like two weeks later I got fired from my job.
I hold no grudges my ex girlfriend for that, I was far from an amazing partner, but I was now living in a new city where I knew basically no one, with no job to get out of my apartment, with no family nearby, and no money to artificially figure out ways to entertain myself. I had never in my entire life before that felt so alone. Days simultaneously felt ridiculously long and yet time seemed to also lose all meaning and it would feel like a week would go by without me even realizing it. When a job interview would come up, it would be the highlight of my week, primarily because it was an excuse to talk to another human.
I had friends from Orlando, of course, good friends even, and I would chat with them on Skype, and I would call my parents over the weekend, but of course they all had to work during the day and so most of the day I was purely alone.
It was dreadful. It felt like all I could do was apply for jobs, play through games on a SNES emulator, and learn new programming languages to try and make myself competitive in the job market.
I eventually found a job, with coworkers that I actually really liked. There was a married couple that I think liked me but also felt kind of sorry for me, so they would invite me over to their house on weekends to play video games and watch Game of Thrones, and I am eternally grateful for them. They were friends when I really needed friends.
I never really figured out how to be alone; I eventually met my girlfriend (now wife) and I haven't really been "alone" in the same way since then.