Maire,
The bottom line is that you need to come clean with your past and not lie by omission. If the scenario went as you stated with this past realtionship then you really don't have anything to worry about. You made a mistake you made healthy changes. I think anyway.
I have to agree with SpayPets that it is a concern that this incident still affects you to the point where you'd rather not go through with your plan to adopt then share it. It makes me wonder if maybe you don't have a lot more you need to work through before bringing another child into your family.
I had some things in my past that were very painful to have to relive again but it was worth it to me to be a parent to go through it.
You keep asking for ways to get around dealing with your past but I don't think there is any. Adoptive parents go through very intensive background checks. As someone else here mentioned it's better they hear it from you then to find out later through these checks.
Keep in mind that what you tell the SW is confidental. Our SW told us no one would see our homestudy except for him, the agency, and our facilitator. So it's not like they are going to go shout your story from the roof tops or put it in the headlines.
I don't think a mistake in your past like the one you mention will cause you to not be allowed to adopt. And these things show the SW how you deal with hard challanges in life and how you have grown as a result. There's nothing wrong with that.
Judy