Just gotta practice brush control. It seems like you're either dragging the brush away from you or don't have a stabilized motion.
You want to have significant points of contact with the model and your brush hand should either rest on your model or your other hand. You want to pull the brush towards you whenever possible to get the most consistent edge. You should practice making straight lines on just a throwaway piece just until you get some consistency out of one motion with your brush. Eventually you can get enough muscle memory to do it at multiple angles but cradling the piece close to your chest with 3 points of contact on your brush hand and pulling it towards you is the easiest IMO.
You do really have to get a feel for pressure and texture and learn the brush enough for perfectly straight lines though.