Should you forgive a friend who backstabbed you?

Ok so about five months ago i borke my arm at football practice and i had to get surgery on it. So my best friend in the hole world came and stayed over. So he could go with when i got my surgery. Let me remind you im fifteen and i had been friends with this guy since we were 3, i cosidered him as a brother even more than my actual brother. So I get my surgery and he still stays over for two more days. One me, him, and my sister are watching a movie. Im on my bed and really tired so i fall asleep. I wake up an hour later and there i see my sister and him making and god knows what else they were doing. I was in a state of shock, i was telling my selfe get up and kick the crap out of him but i coudnt even get up because i truley didnt want to believe it. So the next day i sat them down and said this happen last night or was i just dreaming. They both denied it but they were both sweating like it was 100 degrees or somthing. I also saw a hickey on his neck. been five moths since we talked