Take this from someone who delayed the MCAT like 4 or 5 times before I gave up and just took it -- you will never feel 100% ready. Now take the tips below from someone whose MCAT finally turned out way better than she hoped, and got her a job as an MCAT instructor 🙂
1) If you're not at least scoring in your target range on AAMC practice tests, you definitely are NOT ready, and need to address your anxieties with the timer at that bottom of your screen. It may be too late to reschedule your currently assigned date, so just walk in there, take your test for practice's sake, and void it when you're done. Don't worry; it won't show up on your record (and even if it did there are 100 reasons people void their exams that have nothing to do with level of preparation).
It sounds like you know your stuff but just need a change in mindset. Try refusing to look at the timer any more than twice during each section. When I started taking practice exams, I kept looking at the timer after each passage and it really got to me. Once I got in the habit of only allowing myself to look 1-2x per section, I almost forgot it was there, and it made it much less stressful.
Also, are you taking your homeworks timed? If not, I highly suggest it. Try taking self-made sections (~7 passages + 15 stand-alones to equal 52 questions total, timed for a total of 70 minutes, I think it was) and timing yourself as you go. Once you master doing individual sections timed, you can build up to doing entire tests timed. Obviously, it's too late to really benefit from that for next week's exam, but this will help in the future.
Finally, do not sign up for the next possible exam date available. You may not be able to make your improvements in time for that, and the pressure of that will not help you. Try signing up for the July dates if they are still available, giving yourself enough time to work on this and bring your practice test scores up to snuff. If you are allowed to cancel next week's date now to do so, I would strongly suggest that, before the July seats fill up.
Hope this helped! Hang in there.