I understand how you feel, and when I was choosing my A levels (in my school we had to do it early) I was very barely confident.
I was deciding between aero engineering and something politics related, because I honestly loved both.
I decided to go for the engineering route (although I'm now applying for physics instead) because it provided a stable backup job in case I don't want to do a risky career (which politics is.) and risky careers like politics and business do not have required a levels anyway. They are about you and only you, which is why they are risky.
I think that's a good rule of thumb, although it's not always true, but anything that is stable and tied to institution = requires A level and university choices, for example engineering and law, but anything that isn't, for example, business, politics, media = gives you freedom to choose.
Now if you are deciding between a "stable" career like law and engineering and architecture, that's a tough one. I will be biased and say that STEM is the best because it gives you the most options (e.g research, engineering job, teaching, finance) whereas law is more restricted to law. But keep in mind that I am someone who really loved physics in GCSE and that was partly why I finally managed to make a choice for A level, and you may be different.
I also should warn you to take only "hard" A levels, and nothing worthless like "tourism A level" or "hospitality A level", because they give you access to better unis (and therefore more doors) and flexibility in choice.
I understand your need to vent, you still have time, so don't worry, keep chugging through life