I just saw this thread and had to respond. I know EXACTLY how you feel. I walked in on my boyfriend of three years with another girl. We weren't living together but I had his apartment key and I was worried because I hadn't heard from him all day. I opened up the door and there he was with HER. I was devastated and can you believe the jerk blamed me because I came in unannounced!! WTF?
Of course I dumped him but it took me a long time to get over it. Three years of memories and love are hard to forget, but it gets better, believe me. There were some days I would go to the bathroom at work and cry my eyes out all over again, then put a smile on my face and continue with my day. Like Gina said, I lost weight and was miserable. But you know what? With each day it gets better. A month after I dumped the loser, I met a wonderful guy. I had decided I wanted to be alone and really didn't want to get into another relationship. I told him this and he was so sweet and supportive. Six months later, we got engaged and now he is my husband.
The point is, at the time I was grieving over the loss my relationship I could never imagine being happy again. I promise you, things will get better and one day you will meet the perfect guy for you. He is out there waiting for you, and when you find him, you'll know. Please feel free to PM me if you need to talk. Nikko and I send plenty of hugs and puppy kisses your way.