Since you say you are normally a positive person, and you family has not just suddenly changed, but has been experiencing these problems for a while, I truly feel that you need to see your doctor and be evaluated for depression. Most docs have a little questionnaire that they use to help evaluate your problem. Remember, depression is not a matter of having a screw loose, it is a chemical imbalance in the brain. These chemicals can get knocked out of whack by a number of things...one you mentioned is the seasonal type, another is premenstral, sometimes a number of minor catastrophes all at once can throw things off. Sometimes, a round of antidepressants is all that is needed...maybe 6-12 months and counseling. Sometimes just counseling, either way, it isn't just a matter of sucking it up! Short term blues lasting a week or two, maybe, but when you begin to lose your energy to carry on with just daily activities, then it is time to see your doc. Your doc will know if you are clinically depressed and need medication or not.
That being said, I will pray for you for strength to get through this tough time, but don't be so hard on yourself, admit that you need some assistance with this issue and then go for it...you are worth it and also don't have to be super woman.
God Bless