Don't try so hard to stay on the lines. Once it is all done being close is good enough. Don't watch where the needle is, always try to keep your eyes an inch or two ahead your hands will follow where you are looking. Below is a close up of border work I did with a stencil where you can still see the chalk a bit. Believe me, I drifted off the line quite a bit but as you can see the overall effect doesn't show it. Especially when you do a lot background work. All your eye takes in is the main motif not each line that makes the motif up.
Border done with stencil and then background pebbling
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