Mate I'm so sorry you're feeling so blue
I completely empathise with you and would feel exactly the same,.... I don't think you would be human if you didn't. The thing is, you WILL have the family you crave so deeply my friend, but with the RIGHT person. It's easy to make assumptions that everything is delightfully happy with the ex and her new beau, however, just because she is pregnant, doesn't mean it's all roses with them. I know you're hurting due to the struggle you both had to start a family, but you know what? Everything happens for a reason, and the time just wasn't right for you at that point in your life. If you and her would have started a family, would it have prevented you from breaking up? Possibly, but for the wrong reasons. If you broke up, it could have caused lots of further issues due to access rights etc. Whatever happened, happened because someone bigger than all of us can see the bigger picture in all this. Sadly we don't see that, we just see the pain we face.
I know it's not easy mate, but your time WILL come, and when you do start a family, you'll have the stable, loving and harmonious relationship with the most perfect woman for you,... and that woman is not her. Try to stay positive, and maybe try to look at it differently. You're probably best out of all of that mess. You say she was desperate for a child for 5 years,... well, I'm sure that isn't a valid reason for her to be with someone!
Chin up my friend, you're a much stronger person that you believe.