Sorry to hear you're going through a tough time. Such a time can definitely lead us to question our self in a number of ways. As long as the questioning is constructive, as opposed to destructive, we have the chance to learn more about our self as we progress through life.
One very important question comes to mind, to get the ball rolling, 'Am I dealing with a physical issue or more of a psychological one?' Of course, whether the core issue involves our body or our mind sometimes one can impact the other, leaving us in an overall state of unwellness.
The list you provide, that involves some of the challenges you face: Disinterested in everything, not wanting to interact with others, concentration is slipping, getting sick and run down a lot, mood swings, issues with concentration, eating a lot. On a few occasions, I've experienced similar feelings myself but for very different reasons. 1) Within depression, 2) with sleep apnea and 3) with a vitamin B12 deficiency. The eating aspect didn't apply to the B12 issue but it did when it came to the depression and sleep apnea. By the way, the overeating with apnea is not unusual as it's to do with the body craving energy from anything you can get your hands on, especially sugary stuff. Our body is an amazing thing, the way it tries to compensate!
Seeing you refer to your parents as being the best ever (you're very lucky indeed), consider talking to them about how you're feeling. Mention to your mum, for example, how you feel run down and unwell and how you might benefit from going to the doctor to get some blood tests done. GPs will generally run tests before or whilst addressing the possibility of mental health related issues.
It sounds like you're definitely struggling, so it's important you don't face this alone. As a mum, I'd much prefer my son and daughter to seek my help and support rather than face such a challenge on their own.
Take care Mads and start that much needed investigation :)