Re-igniting the spark? Well I was in a similar position a few years ago: in a relationship for 15+ years, and all OK-ish but all very familiar and routine, and a bit dull; my interest was taken by someone else, and of course they did seem so much more exciting than my dp. (we didn't have an affair, and I don't even know if the other man was up for it)
So there I was in a dilemma - to end a good, solid relationship which wasn't setting the heather alight, and all on a whim with nothing concrete to go on - or do what? I decided that I owed it to myself and dp to make a go of things, and as someone posted earlier I deliberately focused on all the aspects of dp that I really liked and loved. Not the convenient life-style things, but the really personal things, Like how his eyelashes looked when he was asleep, or what it felt like to have his tongue gently touching mine when we kissed, or the neatness of his buttocks, or the very best of intimate moments.
I kept a journal, and wrote down things where he had made me feel good about something - maybe where we'd laughed about something together, or when he'd asked for my advice / help, or when he trusted me with a major decision that would affect both of us, and compliments that I'd stopped noticing (or believing)
And then (after staring to feel lusting and fully attracted to him) I also put effort into creating time for us to be together - a picnic in a sunny wood (with mush kissing), or 15 minutes before bed with the TV off, and the computers off just talking about something, or trying something new together.
It felt like a deliberate choice - I could either choose to notice and appreciate all the good things, or I could focus on his fixation with a boring hobby, or his sagging belly, and my feelings of being taken for granted. All I can say is that it worked for me and I did rekindle the love and lust, and I foresee us being close and happy and sexually attracted to each other for many years to come (and we're already quite old!).
The interesting thing fro me was that I didn't sit him down and tell him he had to be more sexy, or more caring, or more adventurous, it was fundamentally changed by what I thought about him.