I was once the other woman to an engaged man. He is much older than I am. I knew he was to be married but continued our affair anyway.

That affair has come to a close but I see him often and it's clear he still harbors feelings for me.

I am unsure about my feelings for him, but I am absolutely sure about this urge to tell his fiancé that he is a cheater. I respect her and don't wish her heartbreak in the years to come when he is unfaithful again... It would spare them both a messy divorce.

Wedding is approaching. Should I tell her the truth, perhaps anonymously even, or keep it to myself?

Would you get satisfaction out of ruining his relationship? Are you typically a vengeful individual?

Unless you have proof to back it up, like a child, leave it alone and move on.

She has the right to know.

Although J_9 put it a little stark, I totally agree with him. I don't think you're vengeful, as you said you respect his fiancé, and from the sound of it you respect him too, but you shouldn't be mingling with their affairs, and absolutely stay away. As you mentioned he already has the urge to tell his fiancé, so let him sort it out.

You might not realise it, but her having to hear it from 3rd party sources will hurt her even more and will do more bad than good. If he finds the courage to tell her, they will sort it out, or, perhaps, realise they can't continue, but that is not your business.

I would even advise for you to restrain yourself, set up some boundaries and stop seeing him so often, out of respect for him, yourself and his fiancé, in case he can't do it himself. It will have repercussions one way or the other, and since you're not even sure about your feelings for him I think that'd be fair.