I find that when I'm feeling insecure it helps me a lot to get some excercise, dress nicely, etc. and just do what I can to be the most attractive version of myself that I am capable of (within reason). I'm sure you already know this.
But the thing is, then if you are looking your best, you
know there's nothing wrong with you, and that you have done your part. If he isn't interested then you at least can know it's not because there's anything wrong with you.