So glad to hear that things are beginning to get better, and especially that your husband is now working full-time, and that things are better with the in-laws.
How do people soldier on? Just by living day to day, I think. Sometimes things that seem overwhelming when we think about doing them forever are really not so bad on a day to day basis. Someday, those 3 kids are going to grow up and be on their own...Nothing in this life lasts forever without change. The quote "This, too, shall pass" sums up what keeps most people going, even in the face of difficulties.