So...why is he pursuing his older ex-girlfriend when he feels so "in love" with the 19 year-old?
Because they're enjoying the ego stroke from one OW so much, they try to get it from as many as possible. After all, they've already broken their marriage vows - what difference if it's one woman or two...or 10?
I saw this same dynamic with my WH. In the midst of his affair with OW, he flew to San Francisco to give a talk and stopped in Denver for a night on the way back to see his mother. When his mom went to bed around 11, he took her car and went to spend the night shift with a girl "he knew from high school" who is an 11-7 nurse at a hospital. He was there until 4 am, laughing and joking with her.
When he told me about this, I thought it was the most insane thing I'd EVER heard - especially since he never mentioned her before. When I quizzed him further, he said, "Relax. She's a dumpy, middle-age woman. You got nothing to worry about."
This was about six months before I found out about his affair. When I was gathering intel and got access to his email, I learned that not only had he and RN dated in high school, they'd been emailing each other for months before that visit. The back and forth dropped dramatically after he saw her. As WH is a fitness nut, guess RN's middle age spread didn't live up to his youthful fantasy...