Molly,
Don't jump to conclusion on how your spouse will react, but do be prepared for the worst. There is a whole gambit of reactions and emotions you might expect. You may get initial relief it is not an affair, followed by questins of why you didn't tell her sooner, how long have you known, how can you tell strangers and not me. This is how the news can be just as bad, Either way she may see it as an act of betrayal. This is very shaky ground.
I agree with Steph you should not tell her as a knee jerk reaction. You might assure her that you are having to work out some personal issues which have nothing to do with how you feel about her and that you will talk to her in detail about it soon. You should tell her that you are sorry if it seems like you are keeping things from her but that you need to work some things out before telling her all the details of what it is you have sought out therapy about and that the support group is part of that therapy.
Tell her that you want and need to tell her and that you will need her help and support and once you have disclosed everything you would like her to come with you. These are some things you can do with out haveing to give full disclosure before you are ready. Most importantly you should not come clean while she is angry, You will most likely be throwing gasoline on a fire. Be careful. I have said this before and I will say it again preparartion and timeing are key if there is to be any hope of understanding and support from your spouse.
There are of course no gaurantees that with proper preparation and timeing that she will react positively, but your chances will be greatly increased.
Good Luck,
Cassie