Today is the last time the forearm plank will be tested to the max. Perfect form is more important than time.
This is as much a mental challenge as it is a physical one. Check back on your previous best, add some time to it and hold on to that number as it gets tough. The difficulty can often come in waves and if you hold on or change your mindset it can get easier for a length of time. Also when you have an impressive time to attack it can be daunting from the start. Don't allow this convince you to quit early. 28 days in and you haven't quit yet so just hold on as long as you can. And then a little longer.