You've raised a serious need, Lydia. It's wonderful when students openly appreciate our efforts and when we can see positive results in our students' progress. But we do need some strategies for keeping ourselves going when those external validations of our own worth aren't in evidence.
One idea is to ask students two questions:
1. What am I doing that is particularly helpful to you? (This assumes there is indeed something positive going on, and it may turn up ways you contribute to the student's well-being that you were completely unaware of.)
2. What could I do to make your experience in this class better? (This, too, asks for something positive and may produce some good suggestions. It also helps students realize that some things are not in the instructor's control.)
Another aspect of teacher self care is remembering--and actively working to make it so--that teaching is only part of our lives. I think we need to provide regular nourishing (emotional and spiritual as well as physical) experiences for ourselves outside of the classroom.