Is this person who is getting to you someone who is still currently in the picture? I'm kind of confused reading your description - I can't tell if she offered to paint the picture for that guy when you were dating her or if the painting is something from the past.

Assuming that all of this stuff is in the past, I would remind myself that
1. She fell in love with you, not him.
2. You are with her now, not him.
3. She chose you.

Sometimes people don't make the best choices. Sometimes people decide to have flings that they know won't go anywhere, even if it's not what they'd normally do.