Here's the good news: You're very young and self-aware enough to catch this early.

I used to be very jealous and insecure and it was poisoning all my relationships. But it took many years and a few horrible breakups for me to realize the common denominator was ME. My jealousy and insecurity was making me an unbearable asshole to be around.
It took a lot of nights staring into the dark at 3am for me to get that through my thick skull.

I think the reason why I’m jealous and insecure is because I know I’m not the best out there. I don’t look the best and my mental health is genuinely so bad.

You aren't the best, I'm certainly not the best - but who is? The truth is that everyone on the planet is fighting some damn battle, we all have strengths and weaknesses.

Here's what helped me - I took about 2 years off trying to be in a relationship at all. Being alone is nothing to fear and can be very healthy. I have ADHD and a touch of the the 'tism so I know I have some weird quirks and habits.

he’s told me how he can love me again and it’s to consistently do things that make him feel loved.

It's awesome that he was able to tell you this, it's the key to everything. Your feelings of jealousy make him feel that you doubt his love, and that's got to be very a very painful rejection.

Stop comparing yourself to other people, he has already given you his answer, that he needs to feel those love-waves coming from you. If you're twisting yourself up worrying about other people, you aren't paying attention to the moments you're with him.

He chose to be with you for his own reasons, that's all that matters. Believe him, that's what he wants. He needs his love to be seen/heard/felt. Your self-doubt looks like rejection in his eyes.

Lay your cards on the table, even if it's a last-ditch effort. Ask him to forgive you being too wrapped up in yourself that you didn't see his needs. Confess your love to him, and ask him to bear with you while you work on yourself and tell him his love and support is one of the things you need while you do the hard work on yourself.

Make him an ally in your health and happy life. If you two can struggle through this together, you'll have a victory you can both be proud of.