Didn't want to read and run, I am a divorcee of over 6 years and though not the same situation as I have more than one child with my ex I just wanted to say that it takes two to make a marriage work and you could have kept trying as hard as you would like but if he wasn't willing to (and from your op it sounds like he wasn't) then you couldn't make the marriage work for the both of you.
This sounds more like you wish that you were able to give your child a sibling then it does you wish you had stayed married. And you need to be thinking of them separately as one does not necessarily mean that you would have had the other.
Try not to be so hard on yourself, your marriage ended and from the sounds of it, you weren't to blame. Don't compare yourself to his new relationship. Yes he had another baby with his girlfriend but that doesn't mean that they are happy, or that he even wanted to he might have done it to keep her happy who knows.
I get the grief part it doesn't hit me as much now as it did in the beginning but even now when it does it still hits as hard. I grieve for what should have been if my ex hadn't been abusive or had been able to stay faithful.
I don't think I will ever have another relationship whilst my kids r still young and I've learned to make peace with it and realise we have each other that's all that matters and enjoy watching them grow up. And more importantly I've learned to enjoy my own company and all the positives there are to being single eat what you like when you like, go to bed when you like, don't have to put up with the farting and snoring, watch what you like on TV not have to answer questions like Where you been? What you been doing all the time.
Find the positives in your life now and hold on to them, you may still get the pangs of grief for your marriage and what you thought it should have been but you have to keep looking forward.