I was never undiagnosed, but otherwise I know exactly how you feel. Other than saying see a medical professional if possible I don't have advice because what worked for me may not work for you, but I can offer some words of encouragement.

You're not alone. I remember being there. There are lists (that are way too fucking long for people with executive function disorders for some reason) of coping strategies everywhere, so maybe switch it up and get some novelty. If possible get in therapy before the depression. It's fucking hard but you can make it. I believe in you.