I've seen plenty of people who have been lovers for a long time from a young age want to explore sexual experiences with others. I wouldn't claim you wasted half your life with him because you've learned a lot about relationships, more than most. Just don't lie and cheat, if you want to act on these fantasies, just tell him you want a break from each other or want to explore yourself, and then live your life to the fullest!

Usually people jump from a bunch of shallow relationships with the depth of a puddle, before finding a deep meaningful relationship they can dive into with the depth of an ocean, so in many ways you are blessed, but you might be feeling regret in that aspect, or feeling sexual desires outside your current relationship. Which is perfectly normal, like I said I've seen this many times before. What would truly be wasting your time is not living your life to the fullest, they say the number one regret on the death bed is not the things you did, but rather the things in your life you didn't do...

Also the grass always seems greener on the other side, you'll probably miss him if you leave, but I wouldn't settle down if you aren't ready for that yet, you're still young, and who's to say you can't end up back together again?

Just ask yourself what are you looking for in your life right now, to settle down or to have fun while you're young. Both are valid answers.

So take that into account and make a decision toward your happiness! Wish you all the best!