How to decline job offer I’ve accepted

After months of job searching and applying I’ve landed a great entry position at a large healthcare corporation. The position has great pay, benefits, environment, ideal-flexible hours, and would look impressive on a resume, etc. I’ve ruminated the days before I received the job offer and how this position would fit into my life.. after receiving the offer, contemplating again, I accepted thinking to myself that I’d “figure it out.”

Now I’ve realized that balancing work, classes, personal life, socializing, and spending time with my SO would be exhausting. I’m now regretting accepting the position because it requires 32 hours of work and I’ll be taking college courses part-time (3 classes). I’ve also been going to school part time and working part time for so long that maybe its time to pivot and get any part time job that will be flexible and focus on going to school full-time.

I feel silly because I calculated everything from my awareness of feeling desperate for a job, being overly ambitious with my schedule, work-life balance, and yet I’ve made an impulsive decision. In the future, I’ll talk things through with someone and give myself more time before accepting a job offer. I also feel bad towards the people that have put in effort to get me on-boarded (I understand its their job and we’ve only just started). However, I’d rather tell them now respectfully instead of continuing with the job, finding the hard way that it doesn’t work with my schedule, and have wasted their time even more.

One of my big concerns is that once I let them know, will it affect my future in the case I’d like to re-apply to the same corporation in the future? I plan on being respectful, granted its a large corporation, time will have passed, and perhaps then I’ll have better qualifications. I plan on respectfully thanking them and giving a somewhat vague reason.

I’m just looking for some advice on what to tell them or any other advice. Perhaps some reassurance too :/ Thank you!