Well, consider why she has never married, you keep going back to that. Maybe she doesn't really want that. Some people don't. I'm sure some part of your brain is trying to convince yourself that she never married because she is waiting for you. I've done things like that before. I've wanted someone so bad that I invent reasons for them waiting for me in my own mind. That's easy to do. But it's not practical.
You don't nessessarily have to live your life completely without and cut off from all communication from her. But I think it would be beneficial to you if you would let go. I really don't think "Would I be doing her an injustice by getting back together with her?" is the right question you should be asking. From an outsider's perspective, that sounds like you wanting, wanting very much so, to consider her feelings and how it would affect her, but still really thinking of what you want. You want to be with her, and so you include that want in your thoughts on her.
If you are almost 40, and you had been with her way back in college, I really do think that it is time for you to let go. Normally, I'm a very romantic person, and a very firm believer in keeping strong relationships, and working with eachother and loving eachother, but yes, you did break up. If the relationship was years ago in college, you went separate ways, and you were even married twice, then you did break up with this woman. I really hate to say such harsh things, because I could easily see how what I'm saying could come off as harsh, and I really don't mean to hurt your feelings by any means at all. But take a moment and try to think realistically. She may be single, but you did just say she lives 550 miles away from you. She lives 550 miles away. Not temporarily stays, not visits someone from time to time, lives 550 miles from you. She may be coming to visit, but she will have to go back sometime. The fact that you've posted this message in the "living together" thread, though you two are not living together, and in fact, live hundreds of miles away, suggests part of you that lives in the fantasy world of her being with you.
You have a child. You have a responsibility to this child. What are you teaching this child through example? Are you teaching her to cling to things that happened years and years ago? Are you showing her that it's okay to shun away other possible relationships because of an unwillingness to let go of the past? Are you teaching her that it's okay to have a lack of stability in relationships because of desires lying elsewhere?
Normally, I am a romantic person that believes very strongly in love. I always will. I would love to tell you to pour out your heart to her, go run away to where ever she lives, with your child, and get married and live happily ever after. I would love for that to happen for you, but...I honestly don't think that's going to happen. (Once more, I apologize for the harshness of my words.)
Honestly, I did the same thing you are doing for a while too. With the same highschool sweetheart (how the tables have turned eh? XD) right after we broke up. We had been together for a long time, and he did a complete personality 180. We ended up in the same theatre class on the first day of college, and I pined over him for a while. I even dated his best friend because the two were so alike (yeah...I know...bad idea...). At the time, I tried to tell myself "No! They're different! I'm dating Zack because I really like him!" That was me inventing reasons for myself. When I heard that my ex got dumped by his girlfriend that he started dating right after me, I was actually happy and tried to come up with reasons to be back together again. Then I found out he was already talking to someone else about going out less than a week later. He was saying how he's always really been in love with her, he's never really stopped loving her, etc. That hurt. It hurt a lot, but my friend told me something that I really, really needed to hear, and she was right. "It is never going to happen again." After she told me that, I really started letting go. I didn't stay with his friend long, and I actually ended up with someone else in the same class. I've been with him ever since. I've been very happy and very in love because I let go.
I'm not saying I didn't really love my high school guy. He was my first love of all. But I needed to let go of him in order to move on to someone, who in my opinion, is so much better for me. Sometimes what we really want, is not what is best for us. It takes a step back and a good look at ourselves and the state we put ourselves in by clinging onto things that we really want to realize that.