We all want someone to trust and it's pretty hard to come by, so it's completely normal to have a hard time letting go of a friendship that was very important and who you trusted, but unfortunately all the people in our lives are fleeting. The same and you can always count on at least one person and that is very valuable, all that remains is to enjoy the good times and when they are over, thank you and move on.
I'm glad you have your friend, I hope they will be for a long time. She surely is lucky to have you in her life, and you to have her in yours.