Geeze, the one lady on here is way too presumptuous and quite frankly rude saying that to you OP. Many women have break ups and get back together, sometimes (and quite often with a fling in the middle). It's life, and age has nothing to do with it either. Not everyone has a relationship that is on going. I have a very close friend that after a long term relationship they seperated for 6 months, and when they got back together, the break is actually what they needed and they've never been happier. Of course this all depends on the two people involved, their own mental health and reasons for breaking up and the reasons for getting back together. Life is a constant learning curve and sometimes it doesn't go to plan, that's ok! You learn from it. If this relationship is not going to work you will find out and learn from it for your next one when you're ready. Imo if you leave now you may always wonder if it would have worked out, trust your gut/instincts and go for what you feel is right but also know that if it doesn't work out, that's ok and you tried. You'll be in a better position to know it definitely wasn't working/has no future.
Also, many women have a fling/whatever you choose to call it, after a break up. Emotions are all over the place after a break up. Often this fling reinforces the grass isn't greener and also 'gets it out of your system' as such. You may have felt on a low from the relationship and the fling made you feel desired and distracted. That's ok, you are human. But it sounds like you've decided it wasn't for you and again, you can tick of another life lesson!
You sound like a woman who does know your own mind, is kind to yourself and won't be messed around (ie will leave a relationship if it's not right) so hold your head up high and be you. Life isn't always smooth sailing, but you learn what can get you there with experience. I always believe I'd rather have experience than question 'what if' in life. That doesn't make my life a roller coaster at all! Far from it. It's 99% smooth sailing, but I allow the 1% guide me when I'm not sure and I trust my intuition, and it's always ok in the end. I hope that makes sense! It's hard to explain in a message!
There are no children involved which is great. It's a personal to you, but if you did want children as you are in your 30's then just make sure it is with a man you do want to be with for life. The life lessons help you choose that person as it allows you to test the waters. I'd rather have those lessons than be stuck with a man I don't want to be with for the rest of my life.
As for telling him, it probably will come out? If he wants to make this work and he is of sound mind it will work, however he may need time and you may need to work it through together based on 100% trust moving forwards. If he hasn't slept with anyone he may feel 'cheated', like you've one up on him. He may not. Only time will allow you both to heal and see if you are meant to be. Just don't let it take too long ie years as you'll have wasted time that you could have spent being with someone who is your Mr Right.