This belongs in a qualified professional's office. But my view is this:

There are two possibilities: she's lying, or she's telling the truth. Look at the various implications and consider their likelihood.

If what she claims is true, it would imply some things about your relationship that, based on your description, seem unlikely. Is it possible that there was a dark side to your beloved and you were just blind to it that whole time? Sure, but it seems like a stretch.

Now consider the she's-lying angle. Does she have a proven history of lying for her own benefit? Has she demonstrated an utter disregard for others' feelings and relationships? Has she demonstrated a complete lack of integrity? So the idea that she made this up -- while knowing you're grieving and it would be the ultimate twisted knife in your heart to make herself feel better in the moment -- isn't a stretch at all. It sounds like a modus operandi to me.

Of course, you can never know for sure, so you'll need to make peace with this, regardless of its veracity. I think that will require faith in your beloved.

I'm very sorry for your loss.