My mind stuck with questions "why" after being cheat

I broke up my ex 3 months ago. It was not a long relationship however it is and will be a scar in my heart as my ex lie and cheat on me.
I met him at work. He worked for my company business partner. He approached me first then tried to win my heart then he made it. I totally trust him when he said he was still single and hadn't been in a relationship in past 2 years.
Then I found out he already had fiancee in his home country before he met me. Everything he told me was lie. His fiancee didn't know our relationship until I told her. He even talked with about our marriage and our future as if he really loved me. I don't really know why he could lie such things. I was very hurt when I knew that sometime he talked with his fiancee on the phone in front of me but I didn't know (he spoke Hindi and I didn't know Hindi). I feel like an idiot.
Now I feel better after 3 months but the questions such as "why he approached me" "did he actually love me or just play with me?" "why he could treat me like that" still stuck on my mind. It made me hurt sometime. I don't know how I can throw such questions out of my mind?

The best skill anybody can learn is to "let go"..... yes we all want to know "why" but how much does that really help the healing process? does it make it better? make us remember it better?
The honest truth is this, "people suck." And that means "people" don't always use logic, aren't always reasonable, don't always do the right thing. And quite unfortunately, MOST people do NOT do the right thing or behave themselves or are on the up and up. People are only concerned with themselves and what's in it for them these days.

so stop asking "why" because people capable of this do not think like you, or me. they work on a whole different set of rules that you will never understand, and I will never understand. so in essence, asking why and being a slave to "understanding why" just makes it worse.

the only important things here are: he was a bad person to you, he lied to you, and he's no good for you - and now he's not in your life. Be glad you didn't spend your entire life with such a person and are now free to find somebody who is a better person for you. Don't ask why. But definitely learn HOW to detect people like him. That is FAR WORTH more your time than understanding "why" such people do this.