If she is not willing to make an attempt at a joyous and prodcutive (within her limitations) life, then she has broken part of the vows she made to you. (I know most people don't promise to try to maintain a happy outlook on life, and share the domestic burden explicitly, but I like to think it's implicit in the "marriage contract"). I know upholding a commitment is important, but there are times when it is appropriate to break said commitment.
Yes, you may be financialy responsible for her for a long period, but so what. Is any price truly worth your soul? Besides, your leaving may be the biggest favor to her in a strange way - might be the catalyst she needs to get a life. Or to getting in touch with long lost family members, or ....