FWIW, I think it's normal in a long relationship to experience attraction to others - if not, why would the marriage vows include a vow to forsake all others? In the 25+ years I have been with DH, I have had passing attractions to various men and a couple of intense crushes of some duration. None of it was acted on (except in occasional fantasies) and at no point did I want to leave my DH and be with any other man. One of the main points of commitment is not that you never feel attraction to others but that you don't allow attraction to others to outweigh your love for your partner. The way I see it is this: I know that there are lots of men out there with whom I could enjoy spending time and probably enjoy sleeping with them also. I also assume that if I had spent many, many years with any man, then I could be briefly drawn to someone who appears shiny and new. Sure, after nearly 28 years, DH and I can get very irritated with each other from time to time. But, even after nearly 28 years, DH and I still find each other interesting, make each other laugh, have deep conversations and exchanges of ideas, enjoy spending time together, snuggle up in bed, have sex etc. For me, it is not at all certain that I would be as happy and engaged after 28 years with any other man as I am with DH.
Some people do end marriages and settle down with someone else and make a very successful second marriage. I have friends who have done that. But you say you love your husband very much and don't want this. So beware the hormones. The fact that this guy is from your past suggests he has associations with your youth which probably draw you to him. Also (in case it's relevant for timing), I would say that for some women sex drive can ramp up during perimenopause (it did for me, and remains higher now that I am many years post-menopausal). Particularly if you're on the cusp of your fertility window closing, your hormones can go into overdrive on that front. There was a post on here from a woman a while back who had destroyed her long marriage during peri-menopause. She very vigorously pursued an affair with another man and left her husband for him. Her husband initially pleaded with her not to do this. When her hormones calmed down and the dust settled, she was appalled at what she had done and wanted nothing more than to return to her marriage but she had done irreparable damage and her husband wouldn't take her back. She posted as a warning to others. Don't let that be you.
If you are in your 40s or 50s (I'm late 50s and loving it), I think they are a very interesting time of reflection on your life and what things you might want to chance or focus on for the coming decades. If you're in an LTR, it's easy to imagine that the thing you want to change is your partner but, assuming overall your relationship is a good one, it's often things within you that need to be changed/developed.