Looks like you're off to a flying start! Your first trainee quits as soon as they realize that you are their boss? Swing, and a miss! Hopefully, the next few drudges are willing to declare undying fealty to you.

There is an art and a skill to being a good boss. You need to clearly explain what is required and expected while leaving space for the subordinate to do their job in their own way. You can fail either by micromanaging the employee or by not explaining what you expect. There is a definite skill in threading the needle and allowing enough space for the employee to feel like they are a contributor and not just a cog in the machine while being clear what is needed and expected. Of course it’s also possible that your new hire quit for reasons not related to your performance as a boss and trainer. If possible try to understand exactly why they quit so if need be you can improve your own performance.

So hire someone??

Plot twist: that replacement actually works for another department and was placed there temporarily to make it look like they were going to hire a replacement. They wanted you to work both positions all along to save money.

I hope your boss position was at least a good salary bump from the first job even though you're now doing both. My boss was doing the jobs of 3 people before she was finally able to hire me and now she's still doing 2... sometimes bosses get the short end of the stick. Hope you're able to find a new replacement quickly, OP!