A bit of advice from someone who has a lot of experience with this kind of thing: Never, under any circumstances, admit to having cheated.
Women see this kind of thing differently from men - it strikes at the very heart of their self-worth. This is one instance where honesty is definitely not the best policy.
If you're overcome with guilt, then just stop fucking around. If you tell the SO you've been with someone else it'll cause her
much more pain than it will bring you relief. Believe me, there is no redemption, no absolution in owning up to infidelity. It will irreparably harm the relationship and you will
never hear the end of it. Things might appear to be going well but it will always be there in her mind eating away at her, and it will come to the surface at the first sign of discord. It will poison the relationship in many ways, some subtle, some not. At the very least, the woman will never trust you again, and that causes problems in itself.