Sorry I'm late to the party, but I feel like I have to chip in. Based off your post, I see a lot of similarities (and some differences) in our situations.
I'm 20 and have been in full time work for just over half a year, but I don't see friends from school and very rarely see college friends anymore. I know I've lost contact with friends due to a lack of effort on my part and making excuses not to go out. I can't speak for you but I know I just like the peacefulness of being on my own.
I think we all have daily cycles and breaking the monotony can be hard, but I know I'm personally improving.
I'm not at all a big drinker, and I know it can be hard to meet up with people when it seems that's all they do on their weekends. Instead I'm trying to force myself into social situations where drinking isn't an option. I'm joining up with a local football team in the coming weeks, and have been coaching kids football for the last year. Both are great ways to break up the week, be social and not have to drink. I know you're into your football so maybe that's an option?
As for the gym, it has been the best thing I've done for myself in years. I joined up at the beginning of June and have been 4 times a week every week since. I don't speak to anyone there, just plug in my headphones and workout. If I don't go to the gym, I sit at home bored, stressed, and often angry. I really urge you to try and commit to going regularly. I saw a reddit AMA with The Rock not too long ago and he said you have to form a habit of going to the gym even if you're not working out. If he's not feeling up for it he will sit in the corner and read a book, but at least he's still got himself out of the house.
I'm hoping to gain friends through football in the coming weeks, and if you figure out the girlfriend part, let me know Maybe I'll grow some balls and join Tinder
Good luck dude, keep us informed,
Jordie