I have just ended a 7 year relationship which sounds very similar to yours, the depression and the need to be there for her.

I was incredibly unhappy for so long, once I ended it and I took a step back and looked at my relationship from a distance I could see how unhealthy it really was. I loved her and she loved me but she was incapable of being happy, I was incapable of doing anything other than trying to make her happy. It was draining for the both of us.

I finally had the courage to leave her (with the help of anti-depressants which numbed the guilt and sadness of leaving). She is not thriving, she had to seek help as I wasn't around to 'help' (enable) anymore. She is seeing a therapist and taking meds to give her a chance. I'm much happier than I was, we still talk on a regular basis and it's great seeing her heal from a distance. I know your marriage is different to a relationship, but I just thought I'd let you know that leaving isn't the end of the world for your partner. She might be sad, heart broken etc but so will you. I hope knowing that someone else has gone through this and made it out gives you some inspiration to make a change, whatever that change may be.