It takes practice. If you've been using a tablet the whole time then it'll take getting used to with a mouse.
I've been using a mouse every day for about 20 years. My collegue at works creates all our web graphics with a 2" square glide pad - I couldnt do it but he's used to it. It just takes time, practice and patience.
If your strokes are squiggly, try using shorter strokes, if you need longer smoother strokes, zoom in more. The more you're zoomed out then more small jerks will affect the smoothness of your lines.
You might also want to check your mouse sensitivity, everyone has different preferences but it does make a difference. My bf cant use my computer because my mouse is like crawling through treacle to him. to me, his mouse is like floating on air - its just way to sensitive. The more sensitive your mouse is the more tiny movements will affect it.
Choice of brush is also very important, with coopers pic I chose a sparse brush that would give a fur like texture, if it was a portrait I would choose a soft brush to smooth the skin.