How do I stop an overwhelming sense of worthlessness?

So I keep getting this strong feeling of worthlessness. Like my friends don't need me, I'm not good enough for the people around me/ a relationship, I'm not going to achieve anything. And I'm not one of those people that doesn't do anything about it, I try my hardest to stay positive and surround myself with positivity however I always end up with this feeling. I've been having therapy for over two years now and it's helped but not to the point where I'm okay. I'm kind of at the end of my tether now as I don't know what to do to make myself feel better.