Why am I so stupid? How was I easily blinded by someone I thought loved me?

My ex common law husband moved out in January and it was an ugly blow out but we were able to get along since. We got back together three weeks ago, he was being everything I wanted, I canceled a date with another person for my birthday and went out with him instead, we've been having crazy passionate sex, he's let our children see us hugging, kissing and cuddling (they're 3 and 18 months). My three year old son has had a STRONG positive reaction to seeing us together. Yesterday he asked me to go to a family function with him for Easter, so we could all be a family together and I could meet some of his cousins I had not yet met (they're not a close family). Today he came over to see the boys, hesitantly told me he'd gone on a date earlier today, I said "well I'm nothing to you and you can date other people if you want, I just want to put my effort into this so I don't want to myself" he said "you're not nothing, you're very important but this just came up but I felt bad." He said this to me in the bathroom while I was peeing. He asked me for a hug and said he regretted he couldn't stay later for some fooling around and cuddling. I discovered on FACEBOOK a couple of hours later he has a girlfriend, as of today... about an hour before our conversation. I actually went to his house to ask him and he denied lying to me... of course he did, she was in his bed! I'm normally an intelligent strong woman, how could I be so deceived, hook line and sinker by him? What makes people so blind when they think they're in love? I was told not to fall for him that he just wanted me back because he knew I was interested in someone else but I didn't listen. How does great desire make fools of people so easily?