Many people have trouble expressing their emotions, but a simple thanks, I appreciate what you did, is easy to say. You might feel it comes across as “phony” or uncharacteristic, ebcause you don’t generally do it, but if you make yourself say the wors, after a while it will come naturally to you.
Look at it this way – if you cannot express hakoras hatov to your own parents and mean it, how can you do so to Hashem?