Things That People In My Social Like That I Do Not Understand -- Thank You Notes

I have read -- many, many times -- about the importance of writing thank you notes in both personal and professional situations. I know that all good, well-mannered people write them, and so I follow suit in the interest of not being considered ill-mannered.

Yet I am bad at them. I write one sentence, "Thank you for [X]" and then find myself squinting uncomfortably at the abundant blank sheet of paper in front of me. What else to include? I have already gotten to the point. I find myself struggling to come up with three or four more sentences of content before lamely concluding "Thank you again for X." As though I needed to resummarize the three sentences of content, because otherwise my reader would be confused about the central point.

Nor am I particularly touched when I receive them from other people. Instead, I find myself wondering if I should keep them. Living in a small studio and such, I do not have space to keep people's thank you notes, Hallmark cards, and the like. Yet it feels rude -- a degradation of someone else's effort and caring -- to throw one immediately in the trash right after I receive it.

What am I missing? Am I just bad at writing thank you notes?