What to do.

I need some insight on how to go about this situation. So i got in my first relationship in 2012. We've been on and off ever since because he wanted to to be single and explore. I stood by him the whole time thru him talking to other females it broke my heart because I didn't want to be with anyone else but we were still close so I grew to know him and knew that I wanted to be with him for the rest of my life and that is wait for him until he was ready to settle down. We've had our ups and down, we worked at the same job and went to the same college for a short period.  We broke up in 2013 for good and decided to just stay close friends because he knew he loved me just wasn't ready to settle down. I decided to join the military in 2014 to get away from home and start over. We talked to each other thru mail in basic training. Well we got back together in AIT. I got to my duty station in October and started talking to different people just socially, but begun to like the idea of talking to other people. I went home on leave in December 2014 and i had his mom set up a surprise party for my mom because she didn't know I was coming home. Well he proposed that night infront his family and family-friends and my mom. I said yes even tho I had no intension of getting married anytime soon. Now I'm back at my duty station and I still want to see other people but I don't want to loose my fiancé because I know I love him but he can't stand being away from me any longer. So he wants to get married ASAP, but i know if I tell him I want to see other people I'm going to lose him completely.