How do I overcome my fear of rejection to start a new relationship with my ex-wife?

If it was me, I would just visualize myself as happy with "someone" (there should not be a name or face) and enjoying activities with that person. If you desire re-marriage then see your wedding, clearly, and feel the emotions of it, feel the ring on your finger, but don't see the face of the person beside you. Know that you want the person who is best for you, and you are best for. Want the best possible outcome in a relationship. Allow the universe to either slip your ex into that place, or someone else. You can't force anything when it comes to another person, this could backfire on you. If it's right, and good for both of you, then it will work out; if not you will move on in heart and mind. I wouldn't dwell on the feelings of rejection, or on the feelings of wondering if it is right. Imagine what is best for you and let the rest fall into place.

This is an answer straight from Florence Scovel Shinn about this:

For example: A woman came to me and asked me to "speak the word" that she would marry a certain man with whom she was very much in love. (She called him A. B.)

I replied that this would be a violation of spiritual law, but that I would speak the word for the right man, the "divine selection," the man who belonged to her by divine right.

I added, "If A. B. is the right man you can't lose him, and if he isn't, you will receive his equivalent." She saw A. B. frequently but no headway was made in their friendship. One evening she called, and said, "Do you know, for the last week, A. B. hasn't seemed so wonderful to me." I replied, "Maybe he is not the divine selection - another man may be the right one." Soon after that, she met another man who fell in love with her at once, and who said she was his ideal. In fact, he said all the things that she had always wished A. B. would say to her.

She remarked, "It was quite uncanny."

She soon returned his love, and lost all interest in A. B.

This shows the law of substitution. A right idea was substituted for a wrong one, therefore there was no loss or sacrifice involved.

Jesus Christ said, "Seek ye first the kingdom of God and his righteousness; and all these things shall be added unto you," and he said the Kingdom was within man.

The Kingdom is the realm of right ideas, or the divine pattern.

Jesus Christ taught that man's words played a leading part in the game of life. "By your words ye are justified and by your words ye are condemned."

Florence Scovel Shinn Game of Life

Speak the word into reality that if your ex wife is the divine selection (the right one for you) you can not lose her, however if not you will be blessed with her equivalent that is the right one.