there are a few different ways in which a map could be considered "too hard"
- in this case, the most relevant of these is that it sounds like the map is too hard for you to be able to judge the gameplay, so you won't actually know what you're making without getting someone else's opinion on it. there's nothing outright wrong with that, per se, but making maps you can actually play definitely makes it easier to improve as a mapper, especially early on.
- in extreme cases, a map could also be too hard to have an audience if it's so hard that not even the best players in the world can enjoy playing it. the map has to be incredibly hard for this to be a concern, though, and some might say that it doesn't even matter; the playerbase is getting better all the time, after all, so the map will eventually have an audience. I'll let you form your own opinion on whether or not this is an issue.
- a map could also be too hard for the song. the threshold for this obviously depends on the song, but it's also fairly subjective: different people can easily have different takes on how intense any given song is. however, I'm willing to bet that most people would agree that it'd be overkill to, for example, slap constant fullscreen jumps on a really calm piano track. on most songs, though, if you try to push the limits of difficulty, the map will stop making sense as a map before it starts to feel "too hard for the song."
as for determining the difficulty of a map: if you mean star rating, you don't set it yourself, the game calculates it automatically.
if you mean the name of the difficulty (eg. Normal, Hard, Insane), the
Ranking Criteria establishes some rough guidelines for what kinds of difficulty elements you can use at each level. this is the official way to determine which level a map belongs to, although
you can also use star rating as a hint. do keep in mind that SR may not be accurate until you finish the map, though.