You might of fallen out of love with your wife therefore loving someone else, or you have fallen in lust with someone else while you love your wife. Addict has the right idea imo. Goes for the majority of people is that's how most are wired.
Others claim it's not common and can happen. You're going to need to be really open with yourself about it, and can't keep it a secret. Otherwise you're just cheating and having an affair. Decide what life you want. Just read the articles below. Majority of the time though, it doesn't work out as the women you fall in love with, won't want another in the relationship. My guess is you're having fun with this new person, but on a physical level.
My advice, pick a sail boat and go with one to the final destination. You'll likely sink or fall off trying to steer both.
https://www.askmen.com/dating/dating...same-time.html
https://www.psychologytoday.com/blog...mitted-another
https://pairedlife.com/problems/A-ma...-the-same-time
As with the ladies here, take the advice and don't do it.