Is it possible that you have Obstructive Sleep Apnea (OSA)[0]?
Being tired all the time and not being able to concentrate are prime indicators of sleep deprivation.
I'm biased, as when I was in my late 20s, I was pretty much a walking zombie: tired all the time, unable to concentrate for long periods of time, cranky/irritable, etc.
Then, while on vacation with family, my brother noticed that I was gasping for breath while "sleeping" and suggested I get it checked out.
A sleep study[1] (onsite polysomnography test) showed me waking up 270(!) times over the seven hour test.
Since I was waking up (the pathology is, 1. fall asleep; 2. stop breathing; 3. wake up; 4. rinse and repeat) before getting to deeper levels (stage 3/4) of sleep, I wasn't getting the rest I needed.
Once I was diagnosed and started using a Continuous Positive Airway Pressure (CPAP)[2] machine (efficacy confirmed by a second polysomnography test), I slept through the night without any apnea events. As a result, my life changed dramatically. I had more energy, was able to concentrate, wsa more active, calmer and better able to handle issues/problems.
Not saying that's what's going on with you, but after 19 years of treating my sleep apnea, I've never looked back.
I'm not saying that's what's the issue here, but based on your description, it might be useful to investigate this possibility.
I hope you find your way through this successfully!
[0] https://en.wikipedia.org/wiki/Obstructive_sleep_apnea
[1] https://www.sleepassociation.org/sleep-apnea/testing/
[2] https://en.wikipedia.org/wiki/Continuous_positive_airway_pre...