Why does my corporate job feel like a jail?

Because you don't really know how bad jail can be. You have the option right now to do things, and no matter how little time you feel you have you can get one hobby, talk to one friend, or just go for a run. Take vacations every three months, extend a couple long weekends by taking another leave just around it. Talk about things unrelated to work there, listen to others' stories and learn from them. Corporations will always try to convince you they are your first priority, but they aren't. You just have to convince them that they are, do things on time and plan ahead. But once you're done, clear your head.

Do you have an identity outside work? Hobbies or some mission or passion that you pursue? Something that makes you put in active effort - and potentially help derive happiness from. This activity or whatever it is can be what you do outside work hours to override any bad days with good emotions to sleep and the next day. I didn't have any of this and it hit struck me during the covid lockdown time, and made me start working towards some aspirations. Now this is super hard, bringing sturcture to this activity, and doing the same boring thing regularly - can be gardening, or playing turf football, or lifting weights. The struggle with non work hours is that there is no structure or roadmap untill you yourself actively work to plan and enforce it. Whereas for work hours, there is a contract, incentives and social sturctures around in places as checks for you to follow what to do/expect everyday during work hours.

Where do you work?