Being cheated on changed my life, some support and friends would be nice

Hello! It is a bit scary for me to be vulnerable and share my recent experience but I hope that some good and support comes out of this.

Four months ago my first love and partner of three years cheated on me and abandoned me; never taking accountability for what they did, never gave an explanation, and never answered any of my questions, they just disappeared. They night they cheated they sent me a message (while at the girl's house) saying that they "love me deeply and truly" and that we would "always" be together. The next day they ignored me the entire day while they were at the beach with the girl, they deleted the posts of us together on social media, and was active but ignoring me. He left our three year relationship for someone that he only knew for three days...

It felt like my world fell apart. He is now dating the girl he most recently cheated on me with and is pretending to be super loyal and in love with her. They both are posting and she thinks she is so lucky to have found him and he is playing victim to what happened. It is unbearable seeing them together and seeing him happy after he destroyed my happiness. I blocked him on everything but I cannot help but occasionally stalk on fake accounts.

I found out after we broke up that this more recent betrayal was not the first but the third! He had cheated two times prior with two different girls. I was devastated. It felt like my world completely went upside down. It felt like our entire three year relationship was fake, that our passionate love was fake. I was with him since I was 17, we went to prom together, and we were pretty serious. He was my first kiss even. We planned on getting married, having kids one day (even naming them), and he was even looking for rings were how naively serious we were.

The worst part of it all is that he is gaslighting me, denying when we dated, lied about why we broke up, denying that he cheated, denying that he kissed me, saying we were "on and off," saying I am "crazy," making wild excuses to his new gf about what happened, and having the audacity to tell everyone that I cheated on him! It has been incredibly hard for me to deal with being gaslighted by the only person I have ever loved and still have love for. I have not talked to him since the day he cheated and abandoned me. The last thing he ever told me was that he was "sorry he wasn't good enough."

Also, my self-esteem has taken a major hit after his infidelity. I have always been a confident person but after what happened I found that I do not even like myself. I doubt myself, doubt my appearance, and more. I even find myself trying to bring myself down to be more like the uneducated girl he is with now. I wear more makeup now and even want to change my hair color to that of the girl he is with now. If anyone has any advice for self-esteem rebuilding after infidelity I would be grateful and happy to hear it.

This has been so devastating for me. I've lost an unhealthy amount of weight, cry everyday still, have nightmares, imagine them together, get nauseous thinking about what happened, uncontrollably check both of their socials, have a hard time focusing, have a hard time falling asleep (imaging them together), and cannot stop thinking of him all the time. I feel like I will be alone for the rest of my life. I recognize that I am dealing with post infidelity stress disorder (PISD) but honestly just need support from others who have, unfortunately, been through a similar experience. I do not understand why it has been so easy for him to "unlove me" and forget our three years together in four months and be with someone new. The thought of being with someone new makes me uncomfortable to this day. I've been in a dark place and making friends might help. If you read this far I appreciate it! Take care please everyone.