Often when I actually do the thing, I realize it is harder or feels worse than I was consciously feeling. It makes me think, "Oh yeah, no wonder I was putting this off. This is hard."
An example of this is decluttering, which sounds easy but in reality involves making a lot of difficult, often emotional decisions (i.e. coming to terms with the fact that I'm not actually going to use something any time soon so it would be better to get rid of it than keep it hanging around taking up space).
I'm in the middle of a big declutter at the moment (actually not only decluttering but moving rooms around) so I can totally relate. Remembering things you once wanted to do but ended up on a shelf, can really suck, especially if the conclusion is "actually I'm never going to do it, so might as well donate it".