Thanks for your post. I can see you've got lots of support here so I'll just go all clinical on you instead.
Part of making a diagnosis can be quite complicated, but ultimately it's meeting a bunch of criteria (like a checklist). Here is the one for depression just to give you a bit of an example - https://bluepages.anu.edu.au/index.php?id=diagnosis How people make a diagnosis can vary- psychologists or psychologists might just ask questions, get a bit of your history, or ask you to fill out different questionnaires.
Often people with depression (I'm just using this one as an example) can actually have times where they are not depressed in that they don't meet the criteria anymore. I've had depression for years but in reality it's probably been off and on. This is also part of why people can have different diagnoses - simply they just might not meet some criteria or they might start to meet other criteria.
A psychologist said to me once that if we all work hard enough, all of us will find a diagnosis somewhere. I don't say that to undermine mental illness, but just to share that at times we all may 'fit' with a different diagnosis.
I hope that this helps a little. You can also ask all of these questions to your own psychologist/psychiatrist too so they can explain more in detail.