EP this is a very complex matter that delves deep into the subconscious of many people who have repressed and unresolved emotional issues, that often revolve around guilt and they manifest themselves in the conscious mind as stress....
There's a few very simple questions to ask yourself that will make your decision easier but the most important is "is there anything to be gained by telling her?"
Ok so you feel guilty about things you've done in your past, but you must accept the fact that they are now in you past.........
Your right, feelings of guilt can be released by talking to someone and telling them what you feel guilty about, but you're new lady is not the one to tell......
If you don't tell her then you again feel guilty of lying to her...... Tough choice........
Really neither of you will gain anything by telling her about your punting, you may feel better for being honest, but the benefits of being economical with the truth far outweigh the damage that may be done.....
If you think the guilt that you feel from things you've done in your past in hindering your progression, either in your own life, in moving forward in a relationship, or both, then talk to someone......... but not her........
IMO, you won't gain enough by being honest with her to risk the damage you may do, I say you may do, because you could be lucky enough to have found a wonderful and understanding woman who may not judge you for things you've done in you past, after all you weren’t going out then, but if she really didn't care about your past she probably wouldn't have asked about it, or, she may just be asking because she may be feeling insecure about the lack of, or amount of experience she has......
In conclusion............ I wouldn't.....................
Sorry, I went on a bit there.................And good luck................