It was so easy to make friends during my undergrad because I lived in the dorms. But I lost half of them due to a messy situation with my ex girlfriend, and the other half doesn't live in town anymore.
Now I'm 22, working on my Master's, and I feel like I have no one.
I have lots of friends I would say and I still feel insecure every day that I actually dont have any friends.
I've recently come to realize that I need to be comfy with the ebb and flow of friendships and that it is in fact ok to be alone for a while, but it is good to find people.
I would give yourself some credit, this is the first time you have had to rebuild, and it sucks; but just see it as time to figure out who you are when you're by yourself. You will look back at this time and cherish the time you where alive when you had no comittments to friends what so ever, this is who you are uninterupted.
Its the best time to travel, or if you're in study immerse yourself in reading theory and joining local renters unions and activism groups, which are also a great place to make friends. This is also a great time to develop your own culture and hobbies in depth so when you do get to meeting new people you have some cool shit to show them lol.
Good luck comrade, be kind to yourself.