I had a phase of very low self confidence last year. What really helped me is to go and seek out social interaction. Sometimes when it's just you listening to your own thoughts, you can enter a spiral of negativity.

In my case, I knew that rationally, I have no reason to feel sad - I'm in a good place, I've got a job that I enjoy, a comfortable lifestyle, etc. But my problem wasn't rational in the first place, it was emotional, so reasoning it out didn't help me at all. If anything, it made me feel even worse. Outside perspectives were the most helpful in getting me out of that mindset.

I saw the lengths to which my friends would go to maintain their relationship with me, even if I would occasionally snap at them or be unreasonable. Eventually, this convinced me that I did actually matter, at least to them - I had given them plenty of reasons to leave me behind but they chose to stick with me regardless. This ended up significantly improving my mood, and I now have healthier relationships with my friends, stemming from a healthier view of myself.