Need help understanding life and love

Bear with me I am new at this. I guess what I am seeking is input from people to help me understand how you can give your all (love and time) and be going along fine (at least you think so) and then all of a sudden the one you love just drops you with no answer or reason.
I recently broke up with a girl that I had been living with for over a year and I thought everything was fine and that we were doing ok. I always thought of her before myself and made sure we did things that she liked. We went thur the holidays and everything was fine. Then in late Jan she said that it was over and all she told me was that I was not challenging enough for her. I ask her if she was seeing someone and she said no. I have since found out that she was and have asked her when it started and she says she is not but I know for a fact she is and is moving to texas with him.
I guess my question is how do I let this go and go on when everyday it is on mind and is all that I think about (all the unanswered question and wondering about everything). This is really bothering me alot so much that I am scared to try to find someone because I wondered if it will happen again.
It is starting to effect my relation with friends family and work( I have moved to a small town away from home where it is hard to meet alot of people. So about the long deal just could really use some insight on how to get over this. By the way this was my first very serious love.

Guest,

She actually said you're not "Challenging" enough??!

'Sounds to me as if She's the one with the problem!
She seems to want a Dysfunctional relationship. Anyway, that's what the "challenge" means, to me.

I, for one, would jump at the chance to be with someone who's sensitive, considerate, unselfish....

I know, the First biggie is absolutely the hardest one to get over.
And it may sound pat, but Time really Does heal a heart.

I would even consider seeing a counselor, in case he/she can help shed some light, and make sure this becomes a (positive) "learning experience" for you.

I feel for you, as one who this has happened to Countless times (and it hurts Every time, but never as much as the first.)

Best of luck,

offbeatgrl53pf

Question: Maybe you guys are too submissive?