[First consider, if academic philosophy is the right place to seek advice in this matter. My answer here is not any sort of therapeutic, medical, and/or clinical advice.]
The Present Moment and Mindfulness
These are the following fields of study and practices, of philosophy and elswhere, that encourage focusing on the present moment:--
Buddhism
Exercise
Logic
Meditation
Stoicism (philosophy)
Stoic Philosophy
Marcus Aurelius discusses living in the present moment in Stoic philosophy. I recommend reading Meditations.
"Remember that neither the future nor the past pains thee, but only the present. But this is reduced to a very little, if thou only circumscribest it, and chidest thy mind, if it is unable to hold out against even this." — Marcus Aurelius Antoninus, MeditationsMeditations, VIII, 36.
Buddhism, Meditation, and Mindfulness
The Buddha is recognized in various philosophical canons, including Eastern and Western, as having had insights into the nature of suffering. Buddhism for many, remains as both, a practical and philosophical insight into suffering.
Living in the present moment is emphasized in many Buddhist traditions, such as Zen Buddhism.
"The secret of health for both mind and body is not to mourn for the past, worry about the future, or anticipate troubles, but to live in the present moment wisely and earnestly." — Kyōkai, Bukkyō Dendō. The teachings of Buddha. Sterling Publishers Pvt. Ltd, 2005.
In Buddhism, I strongly recommend this book, Turning the Mind Into an Ally.
"It seems we all agree that training the body through exercise, diet, and relaxation is a good idea, but why don't we think about training our mind?" — Mipham, Sakyong, and Sakyong Miphan Rinpoche. Turning the mind into an ally. Penguin, 2004.
A good book on simple, secular Zen can also be worthwhile. Try to find The Complete Idiot's Guide to Zen Living (please, kindly excuse the flamboyant/harsh title of the book).
Formal Logic, Informal Logic, and Mindfulness
Using formal logic as an attempt to improve mindfulness and mental health is experimental and controversial at this point.
- (https://www.psychologytoday.com/us/blog/what-would-aristotle-do/201403/logic-based-therapy-go)
- (https://en.wikipedia.org/wiki/Logic-based_therapy)
- (https://en.wikipedia.org/wiki/Practical_syllogism)
Essentially, you use heuristics (rules of thumb) to improve your mood. If given a big problem, use the heuristic of "dividing a big problem into smaller problems."
Big problem -> Heuristic -> [dividing a big problem into smaller problems] -> Smaller Problems.
Big problem -> Heuristic -> [focus on what you can control right now, this very moment, this very instant of time] -> Smaller Problems.