Having gone through a similar phase before, with all the stress culminating in a panic attack, I feel I can share my experience and insights.
A startup essentially exists to test certain hypotheses or sets of assumptions, both external (like market demand, venture capital interest, revenue potential, etc.) and internal (such as the roles and responsibilities of each founder, assumptions about founders' skills and capabilities, etc.).
As you proceed, you continuously validate these assumptions against rapidly changing conditions, such as shifts in the AI market, waning investor interest, or the actual impact of your product on the problem or need it aims to address. This also involves reassessing internal assumptions.
As we all know, 9 out of 10 startups fail, making success the anomaly. It's quite common for startups to shut down, so if yours fails, it was the most likely outcome.
We're also aware that there are more factors influencing a startup's success or failure that are beyond the founders' control than within it. Therefore, failure isn't solely the founders' fault.
From my perspective, your internal assumptions have not held up, and the external market has changed drastically. Shutting down under these circumstances is perfectly normal. Taking a break and preparing for your next venture is a wise move. Each startup failure provides founders with valuable lessons for their future endeavors.
So, don't worry too much. Just shut it down and take a break. Given the current situation, your startup isn't likely to grow.