How do you keep rejection from negating your self esteem?
Love You First
This is the truth as far as I see the world of love and romance. The only person who you know that will truly love you for who you are is you. If you need some one else's approval to find the love that is in you heart, then you have an issue that only you could resolve. Once you know that you are a unique and special being, one out of about 7,000,000,000 humans then you should always feel special and good about yourself. This is not to be taken the wrong way, by no means am I a solipsist. Everyone is unique and special and is valuable in my eyes.
In relationships there should be a mutual understanding the other person does not make you happy, they increase your happiness. You happiness is just as important as theirs. If they really love you they would understand that concept. There is no point in becoming over attached to a person because this world is temporary. I specifically said over attached. It is over attachment and the need for another to validate ones worth that lowers of raises people's self esteem.
Once you know who you are then the praises of others or the rejections and insults of others will neither make you too high or too low. So it was not meant to be with that person there must have been a reason. So they do not love you anymore and do not want to call you or speak to you. If they really love you they would be concerned about you well being and be there as a true friend.
Your self esteem is what you think about your self. If you truly love you then you know, it does not matter who thinks what about you. Deal with the reality of life, it is not likely that everyone will like you or accept you. It is going to be there loss if you have good intentions for the relationship. So if some one rejects you and you know yourself to be a person who cares to be good and improve situations and relationships continuously, it will always be the other persons loss.
Even better the person reject you, you could reject them right back. Why do you need them anyway. If they are not critical for your basic needs, food water, air, shelter, health, housing, they are free to leave. 7,000,000,000 people and one person rejects me, I am not going to loose sleep over that, I will still wish them well and hope they find who they are looking for, so I may find the right person for myself. Life is too short to worry about what other people think about you. What you think about you and how you connect with the environment overall is more important.
Lastly, if none of that is comforting, then remember God love us all. How you interpret the word God is up to you, but there is a force of love that is cosmic and that force love all of us the good and the "bad". May you find peace with this answer.