If you reach a point when working on a task where you realise there is a strong possibility you won't achieve it, then consider if any adjustments can be made in terms of cost, quality or time to make it achievable.

Cost: Do you need to invest more money to pay for more resource? Could you pay for a locum to free up the required time?

Quality: Does the quality need to be quite the standard you originally planned? Do you need to deliver everything you said you would straight away, could it be broken down into smaller elements?

Time: Do you need longer to implement something? What was your original deadline based on? Can you talk to other stakeholders involved to see if more time can be allowed?

Looking at these areas separately may help you identify if there is any flex anywhere to assist you in achieving the deadline.