What you gonna say to a guy who has been betrayed by long time relationship?

we had a strong relationship for 5 years and there are some ups and downs in our relationship which is normal...2 months before she left me...we start a professional course and she suddenly felt interest about our male instructor and no need to explained she ditched me for that guy and blamed me for how awful our relationship was...I really want to move on...my brain said me to move on but my emotion never let me out from this feelings...today I finally get to know they are engaged...I don't know how to keep my calm...I don't what to do it make feel better...