Cheating

So I've been with my boyfriend for 2 years now, been best friends for 6! I am completely in love with him, couldn't imagine my life without him we have a 1year old daughter and he is so good to us both!  I felt like he has been distant this past week so today I went and had a look through his phone and found he had been inappropriately messageing girls off a website and he went and met up with one of them and they had sex... I don't know what to do I have been a mess all day it feels like my heart has been ripped out I have never felt like this before! 

I don't know what to do! Help!?