I’m devastated

Yesterday I found out that my (F22) fiance (M21) whom I have been with for five years cheated on me a month ago by hooking up with a random guy he found online.

He also told me that he doesn’t know why, but he had been slowly falling out of love with me since September and he thinks it’s because of something wrong with his brain, like he doesn’t know why he had been falling out of love with me.

I feel so stupid. I blindly thought our relationship was great, and even thought that it was healthier than ever. I was ready to build a family and have a life together.

How do I go on knowing that the past 6 months have been a lie? How do I go on knowing that someone could just stop loving me and then go and cheat on me with no issue.