sadness drains me

I posted here the other day telling my story I thank everyone for the advice but today when I got to school all I wanted was for me to die I was lonely, alone, sad upset and when I saw my boyfriend at school all I wanted to do was cry I held it together and didn’t. I had such a horrible day I am sitting by myself after school in Starbucks after my friend left (my only friend who has been there for me the most) & my ex boyfriend the one who needed space is sitting there with a bunch of girls and I know now that he left me so he could try and get with this other girl. He paid a lot of attention to them and when I watched them sit there I only felt empty and more sad I thought I was going to cry right then and there I held it together to show I don’t need him but I feel like I need him so badly after everything we went threw its over because of someone “hotter” than me I do admit I have a pretty low self esteem when Jr comes to judging my self so I don’t normally have a whole lot of good qualities I would say about myself. Any ideas or advice? and now when I see my ex all I do is feel more depressed that I can’t believe I fucked.up again