I feel for you facing such a tough dilemna Jwolf.
On the one hand if you tell her she may react negatively but on the other hand if she somehow finds out in another manner she will probably react more negatively and feel hurt that you did not confide in her.
Give it a lot of thought before you make up your mind on your course of action and let us know how things work out.
Most importantly do not get overly stressed out over this and drink because of it. Nothing good can come from that.
Peace be with you.