Putting on psych nurse hat. When we have an emotional trauma like the death of a loved one we have to grieve and heal. This takes time. When a physical injury is repeatedly aggravated it takes longer and longer to heal. The same thing seems to happen with major emotional events like the kind of losses you have had. Each new hurt re-opens the previous grief and makes it fresh again. You haven't been able to get one death in the family sorted before getting socked with the next. Add a knee replacement and I'm impressed you can hold your head up, let alone work!
A friend, a pastor, a counselor and/or a physician who can evaluate your emotional wounds and prescribe accordingly can help. Grief is a part of life but can lead to true depression when it doesn't resolve. Trauma to the body affects the spirit and vice versa. You deserve to feel better. And, just like knee recovery, "trying harder" probably won't make you FEEL better without the right care.