How to improve?

I'm 23, a PE teacher, above average looking and in good shape. (I go to the gym every morning before work). But in some situations I have no confidence. It's strange for my brain to comprehend because as part of my job I have to be confident. I have no problem standing up in front of the whole school (teachers and staff) and talking, or on parents evening talking to parents. I also play football and am a confident player. However, when it comes to my social life I am dreadful. I have one small circle of friends from uni (most of whom are in relationships) so I spend a lot of my time at home doing nothing when I feel I should be out, meeting new people, socialising and having a good time. I just don't respond well to these situations when I am out and that is a rare occasion. In my head I go through polar opposites of simultaneously thinking I'm better than everyone whilst having nothing going for myself. I expect better for myself but need to change to get there. I need to expand my social circle and make new friends but I'm not sure how to do this and then I need to be confident in these situations. Seems like a vicious circle. What would you recommend?