Just typed a long reply, pressed reply, but lost it because you'd moved the post, so here is an abridged version:
Go to appointment, tell GP (don't bother with a nurse, I've never known a nurse be able to write prescriptions) you are needing the loo 6 times a night. Refuse to leave the consulting room until they have changed your medication. It is unacceptable to need to go to the loo so often, if it can be prevented with proper treatment.
They might suggest something other than Byetta, personally I would accept. Then take frequent (waking, before & after each meal, at night) blood sugar readings for a suitable amount of time (ask how long it will be before you see an improvement from the new tablets, if it is two days test for two days if it is two weeks test for two weeks). If these blood sugar readings are not within the recommended range, go back and insist your medication is changed again. Repeat this until your readings are OK.
DO NOT accept the response of 'try these tablets for 3 months, come back after your next hba1c. There is no reason for you to have high blood sugar for this long.