I started out with mouse about 2 years ago, it was incredibly painful. I mostly used vectors, though, and let the program draw
painterly lines on the paths. And then filled with paint bucket tool

. So it was less of a painting and more of a composing/designing method.
Learning the program is extremely important for me. Some people are naturally talented, they don't care which program to use as long as they can make lines, and they end up drawing gorgeous art. I, on the other hand, need to use the full extent of the tools available, and need to use them fast and conveniently. One reason I dislike Photoshop, it doesn't allow for as smooth of a workflow as Manga Studio does. Like flipping/rotating the canvas, fast&easy brush size change, key toggling ink amount and brush scale affected by pen pressure. Moving stuff around, layer logic and other stuff is less streamlined in PS, as well as it eats more resources.
Anyway, try to get yourself a graphics tablet, even the cheapest one. You'll never regret it

ETA: I'm using
this one. It's an investment around 50€.