It took me awhile to filter through the different "she"s and "her"s but I think I'm clear on the situation.
A couple thoughts...
She's the mother of three children - did she leave them too?
She became involved with a man, found out he was married, was confronted by the man's wife, and is still involved with the man which tells me as much about the man as it does about your wife.
IMO, both of them are involved in a extramarital relationship because they are filling an emptiness not being filled by their existing ones. She said she's young and is looking for fun. What was her life like before the affair? You want to be her one and only, but what does that mean?
From your post it sounds like there might be an age difference in your relationship that's coming into play. Also, you don't mention the ages of your children. Are they grown and taking care of themselves or young, at home, and needing to be looked after?
Your question of her "deserving" your love is something only you can decide. Turning off one's love isn't as easy as flipping a switch based on whether or not it's deserved. We each love who we love for reasons of our own. If I had to guess I would say that she feels taken for granted in your relationship (as does her lover in his) and is looking for some fresh air beyond duty, responsibility, boredom, and "love".