My opinion? It sounds like you are carrying the torch for your wife and don't want to admit that it is over in your heart, hence your hesitation with the old flame. I can understand that, and I feel your pain, there is no judgement in what I say, believe me. To be honest, I don't think the issue is your old flame, I think it is your wife, and to me, I would tell you to shit or get off the pot with her. I am sorry for the crude language, but I think you owe it to yourself and her to figure out what is going on with you two, because things like your old flame will come up, and you will be denying yourself companionship because you are deliberately leaving your marriage in an up in the air state. My recommendation is talk to your wife and lay it on the line, tell her you love her, that you are willing to work through this, give her space to come to grips, but that if she feels it is over, if she ever had any love for you or still does, to tell you the truth, no matter how harsh it is..ask her if she has someone else, and if so just let you know. You owe it to yourself and her, it could be she still thinks things can work, which would be great for you, but you deserve to know the truth.
Given how you feel, if you meet up with the old flame, I would recommend it be as friends and not do anything, you are too torn and it will make you feel guilty and it won't be good for your old flame, either. I don't think there is anything wrong with having a fling, but I think you need to clear your own head and heart before doing anything, not fair to the other person and won't feel good to you.