I feel like it's my fault that I lost my best friends.
Why do you think it's your fault?
Because I didn't hang out with them often enough.
It's not unusual for friends to grow apart as time passes, but you also have to work to keep your friends. If you never spend the time to talk with them or hang out with them, you can't expect them to want to be friends with you.
I think I'm better off without them.
If you would prefer not to remain friends with them, that's completely fine. There's nothing wrong with choosing to move on from a group of friends. However, in the future, you need to hang out with your friends if you want to keep them.