My heart is broken i feel like im dying...HELP!!

CoNsTaNcE
Ive been with this man 4yrs we were engaged for 8months...I needed some time apart in hopes that he would get his act right....treated me so good aside from the fact he loved entertaining woman online he promised he had changed and mow realized whats important etc. We picked up where we left off after being apart 2 months starting looking for places and making plans...then i find out through facebook he has been with anothe woman we spoke she filled me in and i did the same she told me theyve been having unprotected sex etc. We confront him he says nothing long story short she said she was done with him and he said the same about her then days after i was blocked and when i did get on his page he posted a pic of them saying #itsofficial im so confused and hurt i feel like i cant breathe i wanna move on but i dnt kno how and my mental history is not helping the situation one bit...i feel loke i deserve an explaination when i asked him about it he deleted the post but now he wont return any call or messages for the past 2 days...and shes posting how much shes in love with him even tho its only been a short time was her exact words guess how long 2MONTHS....i kno i have to let go but the question is how??? How do i start?? Somebody plz help me