Creating straight lines does not have to be a cause for grief. There is more than one way to draw a line with paint.
I do the masking tape ritual on occasion. This is where you gently place tape on the surface of your work in progress and paint over one or both sides of the tape to create straight lines. But even then in this situation, sometimes painting the lines can be off. For perfect parallel lines, first measure with a ruler, draw with a pencil, but erase most of it until it is very light gray, otherwise it will muddy the picture.
Another trick is to draw the line first and measure with a ruler, as in the first trick. Then use the same ruler as a guide when painting. To do that, hold the ruler at a 45-degree angle (sorta like partially opening a book). Keep holding the ruler with one hand. Take a brush filled with paint in the other hand and hold it like a pencil. This is a trick of the trade for comic artists. Lean the palm of your brush hand against the top of the ruler and glide the brush across the canvas. If you lay the ruler flat, the paint almost always seeps underneath the ruler, causing more frustration.
For smaller lines, you can use your mahl stick or arm as a steady your hand, the same way you would use the second technique with a ruler. The only difference is in this trick you can hold the brush in several different ways, rather than the basic pencil hold.
Any type of brush can be used for all of these techniques. Use the appropriate brush for what you want to do. Small brushes will give you smaller thinner lines, bigger brushes gives you wider lines, regardless of brush shape.
If you get very good at these techniques, you will have the ability to keep your painting on the easel rather than laying it on a table or floor.