Once I was talking a walk with a friend of mine and she "reprimanded" me for turning my head down when people walked by.

Obviously I was lowering my head and gazed downward without even noticing it anymore, it just became a habit. I was told to hold myself up and look people in the eyes...
I'm a woman, I'm much older than you and I'm still nervous as hell, I also still very embarrassed when I catch someone looking at me, I find it so intimate.

I've been working on this though and it's cute when you smile and look at people realize they are looking and smiling at you, too. (Surprise, surprise, it also makes me notice the cute guys who are looking at me much better now.

). It's still hard and sometimes I forget it, but try to push your comfort zone a little bit every day till you get used to looking at people without feeling embarrassed. Once you have done it very often, you will get used to do it.