Even if she refuses to speak about her past, perhaps some medication can help alter her negative mood. Another idea is for her to begin to address other subjects, which can lead to talking about her past in a more natural manner. Finally, exercise is the greatest anti-depressant. Maybe you can help your mother enroll in an exercise class geared both to people her age and in her physical situation. For instance, swimming is a recommended exercise for people of all ages.
Since your mother is not interested in joining a support group, possibly just getting her involved in more activities would be helpful. Other people surrounding her will likely help her. However, if your mother is indeed clinically depressed, getting her to do anything will be challenging. You need to prepare yourself for some resistance, as depression sufferers generally do not have any desire to do anything. This may force you to practically have to drag your mother to some of these activities, but remember that it will be well worth the difficulty if your mother responds positively and her mood begins to improve.
Thank you for advising all of us to be appreciative for what we have. I wish you much hatzlachah with your mother, and I hope you have the zechus to help her unburden herself.