That did come across as quite harsh cricketballs. As a teacher it is not possible to be perfect 100% of the time and to someone who is a prefectionist, or has anxiety issues, that can weigh heavily. I have seen good teachers go under because they just couldn't see that they were good.
HtotheS you have to have targets as part of your performance management round, but they should be set in collaboration with you not just handed to you. Can you go back to your HT and talk through breaking them down into smaller steps that seem more manageable to you?
Are you under Occ Heath with regards to your return to work? Is there anybody you can talk to about how the job is making you feel?
Sorry to sound blunt and not sympathetic but if you are currently teaching then you should be up to meeting targets (as long as they are not over ambitious for a member of staff with your amount of experience/pay scale) If you are not then you should not be teaching