I fully empathise with your suffering. Obviously you've faced many problems in your life so far and they've made you feel bitter. What you've described about yourself, unfulfilled desires compounded by real life stresses have predisposed you to develop these depressive symptoms. Feeling low, unable control negative pessimistic thoughts and poor image of self are definite features of depressive disorders. You should immediately consult a psychiatrist for proper evaluation and advice.
The depression is caused by hormonal imbalance in the brain centres responsible for our thoughts and emotions. The psycho social features are responsible for precipitating and perpetuating this disorder. The disorder is treatable with medications &/or counselling/psychotherapy. This is not a phenomenon caused by deliberate attempt or a result of sin/curse. Hopefully it will be just a passing phase in your life, if treated properly.