One String, hope you get it worked out, after you have checked all the recommendations above, especially the one about too tight a fit at the nock(which would cause all kinds of problems, like yours) then it is time to try some self diagnosis. If you got a friend with a video camera, digitial or whatever go outside for ten minutes with him, pay no attention to the camera, focus as you would normally when shooting alone and have him film you shooting a dozen arrows from the side, then from the rear right behind the arrow, have him zoom in on this one. And do some from your left side too.Take this inside and sit down and watch, look for your form mistakes, look for what you might be doing with your bow arm at release(torqueing etc) and look closely at your release and arrows as they leave your your bow. I am willing to bet you will find out what the problem is.
Also confirm which of your eyes is dominant, I have had customers in the past that have had this change as they got older due to a stigmatism or other eyesight complication. If you wear glasses confirm there is no problem there.
Good arrow flight is important, but a modest over spine with todays cut past center recurves especially is probably not your problem.