My husband broke up with me and left me with a child. At first we lived together, but he cheated all the time. I tried to love him, forgave him, and he came back to me. But it didn't help. He left us anyway.

You should try to love your wife, do all the "sweety" things for her: gifts and etc. - to beat evil by your good deeds. God will do His work after some time, and she will be changed. Relationship rebuilding depends not on your abilities but on Jesus' perfection. Continue to pray for your wife, confess verses from The Bible that say God will rebuild the ancient ruins and will turn the desert into a blooming garden. This is how God sees your marriage. Ask Him to help you to love her unconditionally, even when it is impossible for you. He does miracles! This miracle can happen with you as well! Trust God.

In my case my husband asked for divorce. I worried a lot, but God gave me lots of friends and life full of different events. Now I have no time to weep and suffer. I try to be happy every day!

Irina, maybe, your question is a bit incorrect. I think you wanted to ask more about why it happens sometimes when we pray for people they still die.
God controls life and death. Sometimes we can't understand why some people pass away when they are young... But God answers our prayers and you can hear lots of testimonies when people supposed to die were totally healed.