First of all, I just wanna say I also feel like quitting sixth form all the time. I've been dealing with some mental health issues and constantly feel like quitting for fear of failing, especially as my parents are paying so much for tuitions and expect me to get a job in a field that I don't want to go into. Just take it one day at a time. That's what I'm doing too. Think about what you're doing before you quit and then later regret it. You are getting 'free' education right now, but just think once you turn 19, it won't be free, so if you do regret leaving sixth form and consider going back in the future, you will have to pay for it, and it's not cheap either. Also, consider getting a side job, so that you can observe yourself doing difficult tasks, and this may motivate you to try studying even when you find it difficult. To become a clinical psychologist, you will need a level 3 qualification so if you are considering quitting sixth form, perhaps try studying at a college, where you could try doing BTECs instead of A Levels, or vice versa depending on what you're currently doing. Or try doing an apprenticeship, there may not be many for your chosen field, but there are definitely a few I'm sure.
But my main tip would be don't quit just yet and consider what your future will look like if you don't complete sixth form, and if you still want to quit, you should think about trying a different Level 3 qualification. Maybe, you could just finish your sixth form years and then go to a college and do a subject you really enjoy, rather than going straight to uni.