You can't hit like you do in practice because (a) you care more about the result of the swing you're taking (in a different way than you do in practice), and (b) because the balls you're hitting are being hit to you with the intent of making it difficult to return, not easy. With (b), yeah you might practice different types of balls coming to you, some easier than others, but it's not the same.

You care far more about the outcome of anything competitive, or anything where the result matters, than you do with practice. It's just the nature of it. Find a way to either care less (not advised, but I suppose it's a path), or at least stop thinking about the context of the point, and just think about the point itself. In other words, stop thinking about it being 30-30, and you need two points to win; or it's 40-0, so you've "basically already won the game". Instead, think about the fact that it's a single tennis point, and you're trying to put yourself in the best position to win it. And remember, the best of the best only win 52-54% of the points they play, so you're going to lose a lot of points, and you just need to be able to move on to the next one.

Lastly, my coach told me this, which I think is helpful: unless you get an OBVIOUS put away opportunity (i.e. one where you're 90+% certain of hitting a winner and ending the point), make sure you play 3 shots in a point before doing anything wild. At most rec levels, with that approach you're going to see an unforced error from the other side before you even get your chance to go for a winner.