Yes, Red, betrayal is something that happens, and the closer your friends the more apt it is to happen, and the worse it will feel and affect you. Betrayal is sad, and it is also complicated. Why do friends betray other friends? Well, there are many reasons, but it is always very damaging!
As Red said, the more you open yourself up the more vulnerable you become, and she is right that you should hold some things for yourself, and only open up those things if you find the right person for your mate. Then it takes a lot of trust, and you take a big chance opening up yourself to another person. But nothing ventured, nothing gained, so if you become close with someone, and fall in love you must take the chance and open up to them, then if the other person is worthy you will be greatly rewarded!
I am the same as you Red, in that I have had friends come and friends go. This is mostly because of changing jobs and places where I went to school and lived. Some of the ones who have hope in Jesus Christ provides a basis for friendship like no other. Truely, "There is no other Friend like Jesus," and all of us who call on His Name are to aim to be like Jesus.