I have friends in the U.K. who are left wondering the same thing about the AstraZeneca vaccine. I'm going to give you the same advice I gave them: the truth is, you received the vaccine over a month ago and are totally fine. If Johnson & Johnson is anything like AstraZeneca (I haven't seen enough data), the risk is higher for individuals under 30 but even then it's still very minor.
I got the Gardasil vaccine series over a decade ago, starting at the age of 19 (the series included three vaccines spaced out over the course of 6 months after each shot). My mother encouraged and even dictated this decision, but literally everyone else on the planet aside from my doctor seemed to scold me with stories about adverse reactions, permanent damage, and even death. Yes, death! Horror stories galore. I'm proud of myself for never giving it another thought, which now seems like it was out of character for that particular time. However, after seeing a close family friend develop cancer from HPV, I decided that the benefits outweighed the risks for me - which may be relevant to anyone afraid of vaccines in general, including COVID-19. Ultimately, Gardasil never resulted in anything negative for me. I've probably done more damage in my life by worrying about other things!
You said you're feeling okay - I would focus on that. Go for a soothing walk, talk a bath, listen to a relaxing song, pet your cat or dog, etc. Let yourself fixate on the positive things around you.