What most people forget is that there are pest, diseases and the British, or wherever you are's weather to contend with when gardening. You have to give your plants the best conditions you can, feed and support them all through the growing cycle, if something is going wrong it should be evident from an early stage, though high winds and enormous first time of growing sunflowers is just hard luck, don't be put off by it.
Incidentally, if you had supported that sunflower and rubbed away the side shoots you could have come close to the British record, which is only 26 foot, though you'd need it to be grown against a south facing wall.