I understand your thinking to an extent...I went through an intense period in my life where I constantly asked others and myself "well HOW do you KNOW???"

Now I have depression. But it came to me rather late in life, so I have a good frame of reference. No cause for it...no tragic happening. It just happened. I am different now than I was.
I don't know if the knowledge of my happy life is a blessing or a curse to be honest...

I do take an antidepressant that I have (in the past) switched and there was a radical difference. I have also tapered off, and there was a radical difference. Now I'm on the one that gets me through the day, but on a lower dose.
I want the happy pill, to be honest. The "I am normal and happy" pill.... just for a vacation once in a while...just to have the assurance that yes, happiness is still out there and it was real, and I had it once. It isn't just a figment of my imagination.

Anyway, I ramble in my own well of despair here.... ignore it.

I don't think there's any such thing as "normal" or "abnormal"..... We are what we think we are. Being here helps because when you say you overthink things, someone (like me) will also say that they overthink things too. So you're not so alone.