I am familiar with how you are feeling. Often times I told myself, "I'll never find anyone as great as her again." Or "She was perfect and I messed it all up."
It wasn't perfect, otherwise there would've been no problems in your relationship. Also, do you honestly think that there is only ONE person in the entire world that could make you happy? Sure, she made you happy during this phase of your life, but we all change and so does our "ideal mate."
Do not continue calling her. Hearing her voice will bring back all the emotions and feelings you held for her when you were together. She'll also want to talk about the new guy in her life. Trust me on this one...it's better not to even pick up the phone than to hear about him.
It's hard to step back and see this as a small period of your life. But when you ARE married 40 years, you will not even remember this girl. You'll meet someone that will make you say, "wow, I am happy to be out of that relationship so that I can pursue this one." I understand it's hard to date. You'll look at every girl and think, this isn't as good as so-and-so, or my old girlfriend never yelled at me for this. But getting to know people will help heal you from this heartache. Just be fair to those that you date, don't hurt them the way you are hurting.
It's hard, but it's worth it.
Matt