Hey Moony. Changing schools can be pretty stressful for a lot of people, it takes you out of your comfort zone. In a sense, you're in a new environment and have to "Learn a new set of rules" so to speak. In that sense, it could be that these new rules conflict with your current views on the world, you were at a different school for a long time, you were used to it, I surmise that perhaps this new environment is confusing you, maybe even on a subconscious level. Overthinking usually happens when you get a small negative sign that cascades into a big negative niggling through. You need to try your best to distract yourself when you overthink, but just incase you need some reassurance, you are DEFINITELY not inadequate as a human being, as for your contributions, they can go so far as a story you write, maybe being nice to a stranger, nice to your family, nice to your friends, I'm sure you're guilty of at least one of those.
Trust me, these are not mediocre reasons for being sad. In fact, most depression stems from really trivial stuff that snowballs and gets bigger. This is normal for your age, I'm not sure if that makes you feel better or not, just know that your pain is real.
Loneliness is the only one of those that seems like an external issue, do you have many friends at school? I find that making online friends is really great if you spend a lot of time indoors.
