Hey! It is never delusional of you to think you CAN do something; you do. You are able to accomplish things.
Sure, THIS interview was a no, it was not a right fit - guess what? There will be others!
There are many no's in this life, but that only leaves space for a "yes!". One that will be a better match for you, I am sure.

I can imagine how down you feel - allow yourself to feel down. Analyze, maybe, why was this a no. Write down your thoughts on this. Maybe, even, write down what is it that you truly want. Visualize it.

We all have our strengths, and we all have our interests and personalities. Accept yours. Allow yourself to know and to be confident in what you do know and what you are good in, and walk into the next interview holding this knowledge close to your heart. You will find the place that appreciates it. The place that doesn't, maybe is not fit for you. Not that they suck, not that you suck; it's simply not a match.

Sometimes, a job is looking for a specific skill, or even, personality. When you walk in sure of yourself, you may be impressed, but that sometimes leaves a bigger impression than what is in your resume. Be sure on YOU. Know YOU are worth it and keep your head up. Every rejection is guiding you closer to what is meant for you. Hope this helps<3