When someone supported you during a difficult time, a useful thank-you names what they did and the difference it made. It does not need to be polished. A few direct sentences are often the most meaningful.
For emotional support
"Thank you for checking in and listening when I needed it. You made a hard week feel much less lonely, and I am really grateful to have you."
For practical help
"Thank you for helping with the move. You gave up your Saturday and made a stressful day so much easier. I really appreciate you."
For support at work
"Thank you for stepping in when the deadline got tight. Your calm help made a real difference, and I learned a lot from how you handled it."
What makes it feel sincere
Use one concrete detail rather than a broad "thanks for everything." Say the impact it had on you, then end simply. For a message shaped around your exact situation, use the appreciation helper.