The age you're at, those young adult years, including back to freshman year, are some of the toughest!! It totally gets better. I had a nervous breakdown when I was 22 from similar angst as yours, I was very depressed at 17 really from the loneliness you're talking about, I had never had a real boyfriend, I remained a virgin wellll into my 20's

, etc etc.
Your brain and even your body are still going through unique events at your age which can cause a lot of psychological and physical problems. Talk to a dr. If she or he doesn't click with you, try another. And another. Get some support. Maybe force yourself to socialize if you can. And BTW, all those confident people you see - many are hiding true insecurities and self-hatred. You are actually not alone. Try not to over analyze it.