If you tie it correctly, you can tie it then brace the bow. Try search for a timber hitch, another name for the same knot. Sounds like you're looping around the wrong part of the string. Make the end of the string resemble a P shape, with the vertical part of the P pointing toward the other end, and the part looping around to make the circle of the P passing under the vertical and extending to the left past the vertical so it looks something like this: -----P (the dashes representing the very end of the string. Now loop the dash (end) part back under the vertical and wrap it around the bottom of the circle of the "P" a couple times so it ends up pointing the opposite way: P------ (NOT around the vertical part, that would be more like starting to tie your shoe, and that will come loose under tension). Correctly tied, it will only get tighter under tension.
Much easier to show in pictures, but this is a quick walkthrough, hope it makes sense!!