How to deal with loneliness after breakup?

My ex left me for someone else, while we were in a relationship, it broke my heart into pieces , she was there of only what i provided but the cruelty of just leaving , for someone else and cheating on me , i did everything for her, supported her, loved her, validated her, was there for her in need, and what she does is to compare to me all the time to someone else, the guy who she's with right now, just comparing , insulting, and i don't know why I stayed, cause I really loved her deeply, but what she has done has left me a big void in my heart and it has left me with nothing but loneliness.

I just need people here, I beg to ask what should i do deal with loneliness.