Apologise for the wall of text below:
Yeah, Not proud of it but I used to do the same thing as you. I cheated on a girl multiple times that I was with for 9 years.
Where did this stem from? It started with me "getting back" at her fro cheating on me 3 months in to our relationship, but then I kept going with it.
Why didn't I leave earlier like I should have? The reality was I wasn't mature enough to understand that I was using that as a crutch for all the stuff that was broken in our relationship, and I was too scared to leave and not mature enough to understand this.
I led the double life, and a part of me started to die because of it. I had to remember everything I ever said to anyone to make sure I didn't get caught out (I was very good at that - why I was able to make it last so long).
But by far the worst of it was that I met some really wonderful women that I could have had amazing relationships with but nothing good can ever start from a lie, and no one could trust someone who betrayed the person they should hold above everyone else.
Eventually it dawned on me as to what I was doing - and I broke up with her.
I went on to date some pretty awesome women, and when I felt like looking elsewhere (not the that's a pretty girl look elsewhere, but the one where you wanted more) I moved on - but never cheated again.
I eventually found the one for me, we've been together for 7 years, married for 5 and welcomed a beautiful little girl into our lives recently. And through all that time I've never once considered being unfaithful (even though it's been offered to me more than a few times).
Mate, If you're crossing that line then you've already made your decision. It's not because you "love the excitement", it's because you haven't found someone yet that you truly find exciting.
Also you know that this secret will always hang over your head and it doesn't matter when it's discovered it will end your relationship by either her leaving you or changing it so fundamentally that it won't exist any more.
Move on, If you care about her then you'll want her to find someone that thinks she's his whole world and you may have the opportunity to do the same.
Not moving on means that you'll spend the rest of your life in a shallow place - never really learning about what true love really means.