I think my partner of 2 years is cheating on me ...

I don't know what to think... I have this awful gut feeling my partner is cheating on me .. we've been on the rocks for a few weeks and we've finally started to rebuild things. He felt like we were growing apart and he wasn't sure if he had a future with me etc but we talked and things are getting better. 

Now the next bit is going to sound really bad but it's not how it seems. He went to bed this evening earlier than me and he had fallen asleep, not set an alarm on his phone and falling asleep with his phone. So I picked it up, set and alarm and as I went to put it on charge I saw messages pop up and snapchats from girls. So I was upset and I needed to know what the hell was going on. So I looked at the messages, briefly and saw that he was texting another girl, as well as talking about our relationship with her!! That for me is horrible because it's nothing to do with her and also makes me feel like shit because he hadn't even talked to me about it when he was talking to her about it. And his snapchats were in the notification bar, I didn't even want to look at them. It there's 4 different girls that I've never heard of before he's snap chatting... as well as messages from porn things etc like babestation I know it sounds awful to be  looking through his phone but honestly I was trying to help him at first with his alarm and putting his phone in charge and then I needed to know for my own sanity... I am a wreck and obviously I need to talk to him about it but I jus don't want too because if he has cheated I don't know if I'm going to be strong enough to walk away because I don't deserve that... I've stuck by him through some of the most awful times and been there for the amazing times. I love him with all my heart and I just don't know what to do. I'm a emotional mess. I can't stop crying, I can't sleep because of it and I just want someone else is opinion on the matter... I'm so sorry for the esssay and thank you to those who actually read this.