How to not struggle with self esteem after finding out there was another girl?

Long story short I started dating my colleague some months ago and then he kind of didn’t want to deal with me anymore and didn’t want to get together and have something serious. Yet he was still flirting with me at work without any outside contact out of work and gave me false hope that once I quit the job we will probably have something serious. He came two times at my work with a another girl in flowers in her hand while I was working and then he admitted he was dating her and liked her more but it didn’t have to do anything that I wasn’t pretty enough or funny enough. In short still leading me on yet he is meeting now this other girl.

Even though I know I am an attractive girl with a good personality and people have told me that, I still feel now I am the ugliest person in the world and unable to loved genuinely for some reason. I know comparison is a thief of joy but I can’t help myself to look at her social media and feel like I’m less worth than she is( I mean yeah partly because of that guy).

I am kind of desperate for advice. It’s definitely not the best feeling in the world and I don’t really know how to cope with it so any advice would be helpful <3