Hello, that sounds like you're in quite a difficult situation! The first thing that I would say is that neither subject is easy. They both have their challenges, although likely in different ways. The main question is really which subject you would be happier studying and where you think it would take you in the future.
To become a therapist, you would need at least a BPS accredited psychology undergraduate followed by either some qualifications in counselling / cbt, or (if you want slightly more career progression) a DClinPsy which would allow you to become a clinical psychologist. This is an incredibly competitive programme to get on and definitely not something to do if you aren't sure! However, a psychology degree is one of the most diverse degrees in terms of its uses. After graduation you would not have to stay within the field of mental health psychology, you could also expand into HR, management, marketing, research, cybersecurity... many doors can be opened with psychology depending on what route you choose to take after graduating.
With a law degree, I don't know as much but I do know that further qualifications after an LLB are equally as competitive as a DClinPsy and require similar levels of work experience. However, at a guess, you would probably get a higher salary faster. Law is not quite as flexible as psychology when it comes to real life applications, but it obviously won't completely close all doors. There are plenty of applications with this degree regardless of whether you choose to progress to further education.
You will likely be able to swap degree course within the first 6 or so weeks of your first term, but this will depend on your A Level and possibly GCSE grades, as well as the availability on the course. Most psychology degrees will require at least a 6 in GCSE maths due to the volume of statistics that is required for BPS accreditation. That being said, you are more likely to be accepted into psychology after starting a degree in law than you are to switch from psychology back to law.
Alternatively, if you are struggling to make a decision you can defer your place and take a gap year to gain work experience in the respective fields to see what works for you. Most universities will let you defer with minimal issues, all i had to do when I deferred was to email the admissions office!
Best of luck in the future

-Kat (2nd year psychology undergraduate at Lancaster University)