i believe my fiancés cheated!

My now fiancé, and I have been together for 5 years and have 2 girls together. I'm a stay at home mum and he works. Recently he came home from a 2 week job in another state. I secretly read his texts to a friend which I don't usually do because the last time I snooped his phone I caught him sexting another girl, but I did and found a message he'd sent asking his mate to find him someone to sleep with and if he couldn't my fiancé would "find one himself" now he didn't stop to see his mate on the way home but reading those msgs still killed me. I confronted him before he knew I actually read his msgs and he denied any sort of convo with anyone, lied straight to my face then once I told him he still denied it and says he doesn't want anyone else and only loves me. Now I believe his cheated before only because he lies so good to my face. I love him but I feel like I can't be with someone I don't trust and although he would never admit it in my mind I believe his cheated before. Anyone else been in a relationship like this? I'm so confused I need some advice, please. 
EDIT *** 
Yes I don't trust him, as much as I want to I can't. Which doesn't help my accusations but what makes it worst is he's constantly doing stupid things that make my mind go crazy! & I hate the fact I would try my best to work things out for our kids sake, but he can't even respect me as his fiancé or mother of his children enough for his kids sake. We're on two seperate pages and I'm only just realising it now, but what makes it 10 times worst is he's now acting as though everything's fine, like I didn't just catch him out on something shady, when I tell him I want out he makes me feel sorry for him and knows EXACTLY how to work it but I don't want to just ignore it and be okay until I catch him out again, it's draining, hurts so much and I just feel depressed but I don't know how to go about it and leaving him isn't easy but I DO NOT want to stay with someone who can hurt me and lie to my face without even feeling bad. :/