Therapy might help you a lot. It sounds cliché, but you can get a lot of opportunities just through confidence, and the opposite is true as well. Right now it sounds like you’re in kind of a vicious cycle.

Oh, that's the reason you're a failure. Sure! Lots of people who were social outcasts in high school go on to become very successful...or mass murderers, which, in a way could be considered successful if success is measured by body count.

People think too much of high school. It's just random kids living in your neighbourhood - what are the chances that your best friend goes to your class? Study things you like and find friends amongst people who like same things. It's sad being an outcast but it is up to you to change that.

You need counseling. Check if your state offers assistance for mental health resources.

It makes me sad that you're basing your current self-esteem on what a bunch of teenagers thought 6 years ago. I hope you can meet some new people, some actual adults, and build some confidence.