Welcome to the forum!
I'm sorry to hear that you don't have close friends at this stage in your life. I can relate to this. It's great that you make an effort to socialise, and the fact that you used to have a steady group of friends shows that you are socially able. Thoughts such as "what's wrong with me?" and "why can't I make friends?" must be eating away at your self-worth and confidence levels.
Maintaining friendships in high school is made easier by the fact that you can see friends each week day. With differing uni schedules and different types of commitments, it can be more challenging to regularly catch up with people, and sometimes even to find common ground. I only have one close friend I met through uni. There is a group of friends outside of uni that I spend time with, but I'm only close to a few people in this group. My boyfriend is in this social group, so I am clearly very close to him! :P My two closest friends moved interstate early last year, which has been hard, but I do see them when they come back to visit several times a year.
I can relate to feeling lonely, even when I know there are supportive people around me.
I'd like to direct you to previous threads about friendships that may be useful to look through:
I hope this forum is a supportive place for you to come to :)