You mentioned you have been fostering for 10 years now, I assume she is not your first, nor the 13 year old boy. I'm thinking, though you didn't say, there are others. So your question is how to get over the negative feelings. Positive imagery. Think instead of all the good you have done for other children in need. Think of where they might be without you. Even if you don't know, you can hope and imagine they are better off than if you hadn't been a part of their lives. Give yourself a big pat on the back, you deserve it.