Like Pfft, here's the voice of dissent. Some of you are actually suggesting that if the OP's DP becomes suspicious, that she denies and tells lies. That is called gaslighting.
I always recommend in these situations that there is a controlled disclosure, but only after discussing it all with a non-judgemental person like a counsellor.
Good people do bad things. Only you know OP, how you think you might respond to keeping this secret. I can tell you that some of the unforeseen effects are that in your guilt, you may end up over-compensating and letting your DP off the hook for things you wouldn't normally bargain away and you may end up seeing him as a victim, which will alter your view of him and his sexual appeal.
Just having this secret could possibly drive a wedge between you and have an effect on your intimacy. There is often a ripple effect with regretted infidelity.
You might think that your DP would end your relationship instantly if you confess all, but often people are far more forgiving, once they have got through their shock and anger. Many people in your situation think that their continued secrecy is noble and that they are protecting their partner from hurt. This is only partially true because at least some of the reason is to protect yourself from the fall-out.
Go to a counsellor in slower time if there is no danger of this coming out. If however there is a chance that your DP will find out from someone else (likely, given the connections) then tell him, because he might be able to forgive a confession, but not a public humiliation.