Parenting after divorce

How can I parent my child most effectively when our family has been divided by divorce? I'm a single mother of a four-year-old son. He goes to stay with his father, who left me when I was pregnant with him, every other weekend. When he comes home, he cries for his dad. This really hurts me because I've raised him alone, and as a result I've had all the hard work (like discipline). I also have two other children by another man, so it's harder for me to give this child as much love and attention as my ex-husband does. To tell the truth, I think he spoils him. After these visits I end up crying and feel emotionally devastated for several days. I would appreciate your help with this.