Torn between very different courses.

The thing to keep in mind is that a degree in forensic science isn't going to guarantee you a career as a forensic scientist. In fact it's probably not even the best degree to go into that compared to a typical basic science degree like chemistry or biochemistry or similar. Ultimately the forensic science sector is not a growth sector - the UK doesn't have a shortfall of forensic scientists, the sector isn't anticipated to grow much (if at all), and there are an oversupply of both forensic science ("CSI degree") students as well as those with basic science degrees looking to move into it. Your odds of becoming a writer are probably just as good if not better.

The other thing to bear in mind is that forensic science work is very routine, almost mechanical. You just do the same test all day long on different samples, but have to maintain extremely strict QA practices to ensure the results aren't spoiled some way. Forensic scientists do not interepret evidence from crime scenes - they just purely objectively and, as noted, fairly mechanically process it through specific requested tests. In fact if in the report of the outcome of the tests you did indicate you interpreted it in some way, it could well get that evidence thrown out of court because your "interpretation" biased your handling of the sample. Simply put: it's grunt work, for the most part. It's basic lab tech stuff except you aren't even allowed to think about it.

So between those two factors I would honestly really probably not recommend going down that route. I can understand the appeal (way back when as a teenager I was also quite interested in that as a possibility) but the reality of it is pretty bleak all round. You get paid little to do a fairly boring job which you don't even really need a degree for, with very limited scope for progression as well. If it weren't for the popularity of the subject matter in contemporary media, it would probably be considered at best a stop-gap job on the way to a more substantive position for a science grad. This is even before considering what you say about your background and approaches to the sciences have been, which absolutely make it sound like even if you like the idea of the end point, you would not enjoy the process to get there at all (and honestly, would also probably not enjoy the work as it'd probably cause you the same kinds of anxieties).

The reality of things is that most grads do things completely unrelated to their degree subject anyway, and most generalist grad roles don't care what you studied. So you may as well do something you find intellectually interesting for your degree then figure out what you want to do for a job after that, as with anyone else! This may lead you into a specialist role related to that subject - which as you enjoyed the actual subject means you'd probably find that specialist role more rewarding anyway - or very likely as with the majority of grads, moving into a generalist role which may not be exactly what you want but provides the security needed for you to pursue your personal interests outside of work

As an aside, I'd note that the research methods type stuff is actually a very big part of a psychology degree and I think even the training to become a professional psychologist (e.g. clinical, educational, etc). That said I don't think it's something most directly enjoy but it's worth recognizing it is an important part of things and you do need to at least be prepared to continue doing that kind of work, and doing well in it, if psychology is your goal as a degree and/or career.