My chain slips when I use an inappropriate gear, and the chain loosens when changing gears (most of the time). What should I do? As the title says, it seems like my chain slips way too often. This happens if I pedal hard, go uphill, or ride at an inappropriate gear (even when not pedaling hard).

Besides that, it seems like my chain loosens when changing gears. I am not talking about it loosening for a bit - I mean it doesn't even fix itself when I pedal. I need to change gears further for it to 'fix' itself.

What should I do?
Thanks.
Sounds like you need a new chain and cog. It is badly worn and can result in your injury.
As well as a service on the derailleur ( whole bike service would be best).