As hard as it sounds and as risky as it sounds the only thing you can do is front up and admit it.
Have you ever discussed with them what you would do in the event of this?
Can I ask you a question?
If he did forgive you and give you the support you need Would you still want to marry him? Is ur using related to the actual marriage, at all?
Hope u work it out - spill the beans though asap.
Peace X