Assalaam alaikum sister,
You have reached the first step - you regret your mistake.
Next step is to ask Allah(sw) for forgiveness.
Then you discontinue whatever wrong you were doing...Insha Allah you will have forgiveness of the Most Merciful.
About your husband-to-be- he does not necessarily have to know about it because for one, it will create more problems for you and him than solve your dilemma.
It is your secret,between you and Allah(sw), keep it that way.
Do not panic, calm down and pray Istikhara
Istikhara: The Guidance Prayer
If in case he questions your virginity, you can give an excuse of it being due to any of the physical activities done in school or sports.
Baarakallah feekum
Wa alaikum salaam
EDIT::
Thanks Starby for directing me here :)