Does sound like dangerous territory. Feelings take long to discharge and some of them stay forever, which is why people sometimes need to avoid meeting their former girlfriends or boyfriends. Attraction is even more inclined to act out of bounds and fall out of one's control.
Like MichaelHowling said, I'd think that good things, lessons learned etc., are just normal human remembrance. Although they still should not give rise to fantasies. The other things you mention may not be sinful or wrong per se but easily lead to wrong things or even result from thinking one or two bridges too far.
All sins that harm marriages begin in the brain, with entertaining some ideas. When we're married to someone, we are no longer free to fantasise about others but truth be told, we are never free to fantasise--only real relations with someone who actually is spouse. For example, while it's obvious okay, well, it's intended, for a man to have relations with his wife, it's not okay for him to "help himself" while fantasising about her 1000 miles away from home. Gotta wait for the real contact when he's back. Even more so does this apply to people we can't marry. And obviously, it doesn't make us unmarried. We're married not just on the outside, but our brains and minds are married to our spouses too.
You might also want to address the causes of your dissatisfaction with your current marriage, which might be tempting you to look inside your memories and speculate about what-ifs. Those things may be signs of trouble.