I agree it is very difficult to trust again after these sorts of things have happened.
I was engaged to my ex for almost 3 years. We were living with my parents and he instigated a massive row between us all, which resulted in us moving in with 'friends'. He got a job as a roofer and was out a lot of the time and was always down the pub after work which used to annoy me but I let it go. We then fell out and 3 days later confessed that he had met someone else.
I was completely devestated, felt like my life was over etc etc. Suffered from clinical depression, anti-depressants for nearly 2 years, counselling.
His best friend was a brilliant support to me after that. He then fell out with my ex too and always maintained that my ex was way out of order for what he did to me.
Anyway, the best friend and I got together 2 years after I split up with my ex and he is now DH (4 years later!!)
It was easy to trust him because he had always been there, through all the rough times with the ex and the depression and so on. I did sometimes wonder when we first got together if he would pull a similar stunt to that of my ex but I just know that he wouldn't do anything like that, and even if he did, the other woman would send him straight back .
Seriously though, I agree that nice things happen if you don't necessarily think about them, or do anything particular to bring them about IYSWIM. Trust is something that has to be earned over time, even in new relationships and I'm sure that in time, you will meet someone with whom you just click and the trust will be built and your faith in men restored.
Sorry that was really long, but I hope it helps