I would like to offer my help.
What you describe sounds like a lot of social pressure and, to a lesser degree, isolation. You do seem mildly depressed and burned out. Truly there has been a lot of discouragement in your life...relationship failures, older parents who are ill or are potentially infirm; plus you have a past history of some depression and not a lot of solid hope about establishing a lasting relationship.
I really try to limit attempts at diagnosing over the web, but your pattern of emotional response is very classic, and typically what is identified with the common mood issue, dysthymia. This is a mild, chronic and nagging depression. It is very subtle at times, and some days the features can be totally absent.
But, the discouragement; the feelings that there are no real answers; the lack of motivation, and feelings that nothing positive will happen (decently) are typical of it. It is very common in midlife and is very common with women. It is also frequently related to relationship issues and the frustration and strain of keeping, finding or dealing with them. I feel that you may have this problem and I also would recommend a face to face assessment to see if this is so. Your statement of "I should" do certain things, such as visiting parents or trying to go out and socialize with others but not doing so is also very typical for the issue.
The best news is that dysthymia is highly treatable. Using cognitive behavioral therapy (CBT) and in some cases, low doses of antidepressants for short periods, the despondency and frustration abate enough to reestablish solid connections with others.
Even if the assessment shows that you do not have this problem, there is clearly enough stress and frustration in your life that support from an unbiased and caring professional would help. You are worth it, and I would encourage you to not see yourself as defective or odd, but rather as a person who has been though a lot and needs some support. Seeking help, even here on this site is sign of strength and it also shows that you want to feel better.
Consider seeking out a therapist. Family doctors often have the best local resources and can often make excellent recommendations that fit your needs. I feel strongly that the support that you will receive can help you tremendously. Steven