When you refuse to let go of the past, the present will pass away without you knowing and the future will never come. The past is history, the present is news and the future is a dream.
You might have reasons to hold onto the past, it could be joyous memory or past failure. It could be sweet moment or broken heart. None must keep you from moving on.
In the past lies moments and opportunities that you will never and can never have. But in the present lies decisions and steps that can and will deliver the dream future to you.