I know the feeling of just wanting to curl up in bed and stay there. I've felt that way myself, many times. What you describe sounds like a classic case of clinical depression. It's a medical disorder; it's not anyone's fault, and it's nothing to be ashamed of. You CAN feel better, you just need help to do it. Talk to a doctor.
I'm so sorry about your daughter. I can't think of a more painful thing than losing a child. However, I can think of something equally painful: losing your mom, especially while you're still a child. Remember, to your son, *you are the most important person in the world.* Don't think of it as "he deserves better than you." What he deserves is to keep you, the mom he loves, and receive your love and support. He is a reason to get up in the morning and go on with life. Even if you feel you don't deserve happiness (and you DO deserve it), you need to get better for his sake.
A doctor may prescribe antidepressants that could help you a lot, but I suggest you also seek counseling. The double whammy of losing your daughter and your husband must have been very hard. It's great that you had the strength to go on and take another child into your home and heart, but if you're still in mourning after four years, you may need help to deal with the grief. You'll never forget your daughter or stop loving her, but she should be a cherished memory, not something that interferes with sharing life and happiness with your son.