I think you have to be honest in your situation. Just think how she might react if you are not hones. You are not having much sex but you are really working on getting bigger. Her assumption will most likely be that you are doing it for someone other than her. Under the circumstances it would be a pretty reasonable assumption. Tell her the truth and it will probably help the relationship. Also, you should emphasize that PE is about you and not her so she doesn’t feel that she is somehow inadequate or unsatisfying to you now.