Unfaithful

What's going on in today's world? Seriously, does the term faithful mean anything to people anymore? I'm aware I'm making a HUGE generalization but this is something that is really common in my life and it's getting a little sickening. Commitment doesn't seem to mean much to anyone anymore. Someone correct me if I'm wrong, maybe I just hang around a lot of people who don't seem to grasp the concept.
I don't exactly know what to say about this whole thing. I've cheated on someone in the past and I've been the other girl. Those are probably the some of the biggest regrets of my life. I think you really need to put yourself in the other persons shoes. Imagine the person that you're so commited to hugging and kissing someone else, calling some other girl/guy before they go to bed, daydreaming about being with someone else.
I think that my choice in guy friends has really led me down this road to where I am. Don't get me wrong, my guy friends are nice enough, but some of them don't have the best track record when it comes to girlfriends and being faithful. I've seen it, I've seen them lie to a girls face, avoid telling the whole truth, find ways to make another girl work into their schedule. It seems so easy. So easy for them to tell this girl a lie, this girl, that they apparently care enough about to be in a relationship with.
So what's a girl like me to do? I just got back with an old boyfriend. Things are amazing, but I'm scared. So, so, so scared that there could be someone else. I know he is an amazing guy, with amazing morals. But I used to think that of some of my friends. So how do I get rid of this horrible feeling? How do I know for sure that he's not lying. He's told me white lies about this other girl before, he got a zippo from her and lied and told me he bought it for himself, we weren't together then and I think he just didn't want to hurt me. He doesn't use it anymore if that changes things. It's just when he tells me a lie about something so small - how do I know he's not lying about bigger things? If he can't tell me the truth about something as little as that - how can he tell me the truth about other things?
Deep, deep down I know he would never do something like that - but society and my friends are all pushing me. I've got all these insecurities like I'm not good enough, smart enough, pretty enough. That he'd rather be with someone else. I hate it because it holds me back. This weekend wasn't as good as it could have been because I felt so insecure.
The worst part is that I've been there. I've been tempted by someone and fallen for their 'charms' when I shouldn't have. I've hurt someone in that way and now here I am scared of having to feel that same pain.