DILEMMAAA??? How to deal with such an issue......?

First of all i ave known dis guy for 9 months now...!...we ave been really close friends and everyfingz beyond perfect, itss unbelievable that we even plan to get married da following september...although we stil in the month of May. however one day he decides to go off with another girl and sleep with her have sex and even go cinema with her...i will admit i am jealous...but the point is we planned to marry. question is how can i be able to trust a guy if i am to be in a relationship with him???...another question should i just let him go, even though i love him so much that i dont fink i even have the energy to let him go ???? another question should i just allow him and let him c the girl, coz at the end of the day we are just close friends ????...:/ :/ :(