As humans, we have been both cursed and blessed by our ability to dwell on the past while simultaneously fretting over the future. Learning from past mistakes is the key to attaining any kind of wisdom -- one could argue that it is the definition of "experience." Planning for the future is equally important.
The problem arises when our worries over the past and future drown out our ability to enjoy the only time we actually have: the here and now. If our "present" is consumed by the negativity of past mistakes and the anxiety over what the future will hold we become lost and adrift. We have relinquished (voluntarily) the only time that we truly have: now.
The Muddy Squirrel solution? Leave the past in the past. Try this approach for life's "dark woods" -- admit your mistake, take ownership of your mistake, learn from your mistake, then let it go.
Nothing seems to cultivate "presence" like time in nature. Go for a walk, a bike ride, a trail run -- anything to help your mind release from the day's "replay" tape and to become present in the moment. Listen to your breathing, inhale earthy smells, watch wildlife, notice the unique qualities of the season. Forget about tomorrow. For 20 minutes or an hour or whatever, just be.
Sounds easy, right? It may not be at first. But, as with any new skill, keep practicing. It will come with time.