Hey, leapingbare
Where did the string break? At the nock end? What sort of tillering string are you using?
I use an 18 strand fastflight endless loop tillering string, with padded ends and middle serving. Overkill? Maybe, but it's NOT going to break!!
Get yourself a pully system on your tiller tree, so you can step back away from danger if something lets go. I also clamp my nonbending handle bows on the tree, so they don't come flying off.