How do I stop sabotaging myself?

I am an introvert and I am a bit of a loner but it is college so now I have to actually talk to people now. I just find it hard sometimes. I fall back into a habit of keeping my head down and pretend I'm busy and when someone says hi, I say hi back but I don't initiate conversation even though I've been invited to. I implied in a previous post that I need to start socializing and expanding my circle but I instinctively close myself. I need some advice on how to curb this behavior. What do I do?