Why am I feeling this way?

Ok right here goes, me and my ex boyfriend where together for 3 and a half years, we split up like2 years ago, but I’ve just found out that my so called best friend and my ex went down on each other when I was still with him!, im with somebody new now an im engaged an di love him with all my heart I would do anything for him! Hes my world!! But the question I have is why am I feeling the way I do becasue I’ve just found out my so called best friend went with my ex when we where still together!!? I it jealousy? Or is it just me being really furious because my so called best friend went with him when I was going out with him!? I have no feelings!! At all what so ever!! For my ex I h8 the guts of him a truely despise him!!, why would I have feelings for him after all he did!? He made me self concious, he made me self esteem go really down hill he use 2 hit me he use 2 stop me from seeing my friends he was a total control freak! He made me go an c a therapist because of all the things he did 2 me! I ended up depressed

Please help me because im feeling pretty confused right now, Thanx xx