I have been a semi-similar position, worked 3 years in non-tech world where I was a full-stack tech lead wizard champion. But decided to go into tech, since I wasn't learning anymore and was tired of being considered a senior in my field when I actually was just senior in my role.

Long story short, joined tech, got wrecked and started to doubt about my skills. Things to keep in mind:

- you got hired. unless they did a shit job interviewing they probably know that you are not Martin Fowler

- you made this career move for that very reason: learning, getting your ego shuddered a little bit

- talk to your supervisor and share your feelings

There might also be some other issues here that are not completely tied to your skills/competences. Some projects are tough. I would not consider working on a codebase that is full of hard coded values, magic and hacks "medium difficulty". In fact, this sounds like a really difficult task as you have to make constant trade-offs (refactor/rewrite/leave it alone).

Anyway, don't be so hard on yourself, we all suck at our job. Communicate with your team.