Afraid my secret will scare potential relationships away.

Sydney Nicole
I'm not dating anyone right now so I guess I shouldn't worry about this right now, but I'm scared that the horrible decision I made in my past (you could probably guess what it is because of where I'm posting this lol) will scare away any potential relationships.
And I have no idea when I should tell a person I'm dating about it.. Honestly it's only a secret I would want the person I'm going to marry to know. The only other person who knows is my ex because it happened with him. 
But I feel like you should know every secret about someone before marrying them so I don't think I should keep it until getting married. But this is a secret I just can't go around telling every person that I date.
I have thought about maybe letting that person know I have a big secret but don't feel comfortable telling it to them unless I know we're really going somewhere. But then what if I finally tell him and he can't look at me the same way because of it?!
I know I don't need to worry about this right now because I'm not dating right now, but I'm thinking about it and I'm afraid. Any advice or has anyone gone through a situation like I'm describing?