We've probably all been there. This scenario is not specific to only teenage girls. Sometimes people leave. Sometimes people hurt us. Sometimes in their pain, people cause us pain. It's usually not intentional, but that doesn't make the sense of rejection feel any better.
As a pastor, I've felt it over and over. As a matter of fact, it seems that nearly each time someone leaves the church where God has placed me to shepherd, I feel that sense of rejection. In some cases it's severe, in other cases it may be milder, but that's directly related to how close that person and I had become over the years. I know that it isn't always personal, usually it's not, but it still feels that way. Sure there are exceptions, such as, if God has called someone into a new ministry and they are leaving to step in to that calling. There's joy in that! There is a sense of purpose and accomplishment and we bless their commitment to ministry and to God! Yet in most cases, people leave because they are looking for something different than what they are receiving, they are offended, they are seeking a position that isn't available, or a host of other reasons.
Here's the thing, I really value relationships. I really value each person. If you know me, I hope that's evident. I also really take my calling in God seriously. The Word says that church leaders, (pastors, elders) 'keep watch over your souls' (Hebrews 13:17). That is extremely serious to me. So, for two very substantial reasons it hurts me when people pull away.
Over the course of my ten years as pastor I've seen hundreds come, and hundreds go. If you don't choose to release those feelings associated with that though, it will begin to feel personal. Honestly, if you focus too much on people's actions it will put you in a downward spiral. It will cause bitterness. It will make you harden.
So, it's not only teenage girls that feel hurt by friendships. I know it's not only pastors either. I'm sure you have experienced similar things.
Here's my point. Love hurts, but love anyway. People will leave you, some will hurt you unintentionally, some will betray you and bring pain just for spite... But choose to love anyway. Don't let the actions of others make you hard. Forgive, and although you might not literally forget it, stop holding it against them. It isn't always easy, but love is always worth it. When we love, in spite of how it is returned, that's when we are most Christlike. Sure, love hurts... but love anyway.