Of course I don't know all the details, but here's my reading of your concerns.
- I get really unhappy in this new job
It's possible, but I think you have to give it some time before you can decide this for sure. Feeling a bit unsettled in a new job is normal.
- I get let go from this job, most likely as a result of being unhappy in it and hence not performing
Seems unlikely. From what you've said, you seem pretty conscientious, so even if it's not challenging enough for you I'm sure you'll pull your weight.
- I talk to my new boss about the problems and they decide it's better to let me go rather than continuing to invest time in training me (I have already said to my boss that I was a little surprised at some of the laid back attitude and was told I needed to chillout a bit)
Importantly, you've said that everyone is nice, even if they are a bit complacent, and they have good products. Assuming this complacency isn't causing safety issues, and it's not making you completely miserable, try to stick with it a bit longer and see if your perception changes.
You can always quit later if you're completely sure, but if you quit now you can't turn that decision back. It sounds like maybe their work style is different than you're used to, but give it a bit more of a chance. This is from someone who has resigned too soon in a new job and regretted it later
Best of luck!