Some advice, please

I've never actually been on a forum before, so I feel a little awkward/nervous/weird for writing this. I don't really know where to begin. I've been feeling mostly very lonely and this has been happening for quite some time. The reality is that I have no real friends anymore. A few years ago, I had the greatest group of friends. I was very close with four of them. I shared everything with them and we'd always call each other and talk. I never felt out of place and I felt like I belonged. I was comfortable and just really, really happy. But then three of them moved away. The next year was very hard for me. People tried to talk to me, but I always felt so disconnected with them. I always seem to put an invisible barrier between us and I just unconsciously push people away. I'm not rude or anything. I'm friendly and I smile and laugh and all that, but it's all on-the-surface small talk. Nothing really strong or deep enough to bond over. My other friend and I still talk a lot, but I feel I'm such a burden, like I'm dragging her down because she's been able to move on and these feelings are just expanding this gap between us. I guess things like these just have a greater impact on me. I wish it didn't. I don't know how to talk to people anymore. I sit with this group of people during lunch, but they're not really my friends. I rarely contribute to conversations because I don't know what to say and I just feel really stupid. My friends who left have moved on as well and they're doing great and I'm happy for them. But what's wrong with me? Why can't I move on? It's literally been a few years already. It shouldn't be this hard. I am way too self conscious, it's embarrassing. I always feel like people are judging me when I talk to them. I'm just a really, really sad person. If I ever do hang out, it's because of that one friend I have. That's why I feel like I'm such a weight for her. If we've drifted apart, she's not obligated to invite me to do things and I feel bad for making her feel like she should. I feel so out of place. I keep trying to remember what it was that made us all so tight before and apply this when I try talking to other people. But nothing really worked and I'm doing everything wrong. I know it sounds ridiculous, but I feel like people view me as this sad, sad, pathetic person. I realize I have low self esteem. I hate my personality. God, I'm pathetic.

Sorry, you really don't have to read all of it. I just really needed to say all that.

Here's what I really wanted to ask: How do you start talking to people, what do you talk about that makes you really connect? When is the 'right' time to talk to start calling them up or texting without it being weird/overly pushy/desperate?

Hey, harrington49.
I'm glad you replied after having taken the time to read my post. I'm really grateful, thanks : ).
Actually, me and my friend are drifting apart. It's partly because we don't really have any classes together. I can't seem to relate to any of her friends or anyone else either. We've actually talked about this a couple of times, but I don't want to trouble her anymore. I get that bringing this up can get annoying so I don't talk about anymore. They did leave a few years back.
The problem is, my school is quite cliquey and I guess you could say I 'hang out' with my friend's friends. Even if I don't talk to any of them outside of school. So it is hard to talk to other people - basically, to switch groups. I hate to say that because it sounds really shallow and immature... so high school.
I just wish I could get over things and get over myself and not feel so invisible/worthless. I want to graduate already and shed this image and start fresh. Is it a lot easier in university to relate to people, to form friendships?