Awkwardness when meeting someone

I don’t know when this has started, but sometimes people who I haven’t talked to for a long time (Don’t know why, just somehow stopped talking to them :() greet me as he/she walks past.

As I was walking to my English exam, a fellow student who I knew somewhere in primary school was riding his bike and said ‘Sup, bro’. I hadn’t expected a greeting at all, and I felt really bad because I didn’t greet him first nor did I return a greeting. When this happens, I feel really bad on the inside. I feel the person who greeted be should also deserve a reply, but before I realise it they are usually behind me and I just keep walking on.

This is really bad. It is probably why I’m not all that popular. Sometimes when I walk and I see someone I know I get really nervous and hesitant on whether or not I should greet them. A simple greeting doesn’t talk much, and usually could just be a ‘Hi’ or ‘Sup’ and it is something that would be considered really friendly and courteous. But I usually just end up looking downwards and as I walk past them I feel so ashamed as there is just this ’silence’.

People I have never really spoken to before greets me at school. I never hardly get the chance to reply, as they walk on past. It is just my bad excuse. It’s my problem.

This also happens to be the case for the classmates I’ve had in the past. During the year we are all friendly but nowadays I am hardly contacting anyone. Maybe it’s just me. No one likes talking to me because I’m such a idiot. And when walking around the school campus it’s like I have never met them before. The feeling is so awkward, I don’t know how to describe it. It’s a sour feeling and sometimes when I am lying on my bed at night and I think of these moments, I just feel like twitching and moving around so loose the thought.

Is this just me? Where can I improve? I don’t want to lose any more friends or contacts this way. It’s all too sad.