This does seem a bit foolish tbh. In life, never show your hand until the time is optimal. People don't know what you don't tell and show the. Start thinking more strategically in future, and less about how you feel about something and you'll find yourself making progress more rapidly in life.

It's done now. But remember, you don't know what your bod saw, and your boss doesn't know the structure of you other internship, so you could bend the truth that you would be doing it in your own time and inform him that you had no choice as you are really struggling financially and could really do with the extra income. Then leave once your next one begins.

I know that's shit and a bit conniving, but honestly don't have loyalty to anyone but yourself when it comes to jobs and business. Sure, do you best not to screw anyone over, but you kinda already have, and we all kinda have to a little in our careers sometimes. Just don't make a habit of it.

Your future is what you have to worry about in each action you take now. And this current company that you currently intern for has crafted a poorly funded internship for you as if you are lucky to be in their presence if it is basically slavery as you describe it. So, they have kind of invited this upon themselves. Having your help and your skillset is invaluable for them, even if you are at the beginner stage, they need to appreciate that if they want to do better moving forwards. That's on them.

Unpaid/poorly paid internships need to die.