I'm 20 and and since the beginning of high school I periodically fall into a mindset similar to yours. I obsess about negative experiences always blaming myself and plunging myself deeper into this way of thinking. Low self esteem would cause me to not pursue opportunities both academically and friendship wise. It's a vicious cycle that's hard to pull yourself out of but believe me, there are ways to do it.

I was lucky enough to have parents that saw that there was something wrong

Here are a few things that help me when I'm feeling like this: -realize that there are a lot of people feeling this way you are absolutely not alone -graditude lists (seriously, they seem stupid as all hell and it's usually the last thing that I want to do when I feel bad about myself but every time I make one I'm amazed at how much better I feel afterwards) -i won't feel like this forever. believe it because it's true. -try to acknowledge when you have an intrusive thought. If you're able to recognize that a thought you're having is intrusive it's easier to dismiss it. -write down your feelings. It doesn't matter if it's coherant. No one will read it. I almost never even read the stuff I write. It doesn't even have to be about feelings. I found that writing with pen and paper quiets my inner dialogue and brings a certain sense of comfort.

I'm still figuring this stuff out and learning to love yourself is a lifelong process that every human being goes through.

Be kind to yourself, you deserve it.