I have done my very best to take time to read and understand your post which is so honest and forthright.
You do deserve a medal, and I am sure your post will resound a thousand times with many who are in your position.
On the one side you have the 'home schooling' process which relentlessly keeps turning out the items to be learnt. On the other side you are a human being working in semi isolation doing your best to achieve the grades you need (I also fully support home schooling)
In short you are nearing your 'RAM' capacity, and you are still trying to shoe horn in just a 'little bit' more. You know you need a break.
The positive you can take from this is that you have acknowledged your head's limits and know from your own experiences that you need to take a break. The fact that you have already considered options of how to schedule your day should be applauded. Later in life you will draw back on this time to understand exactly how you can achieve both. Excellent results and a healthy disposition to tackle those times when the demands way exceed the available time or capability are a life saver.
You really do deserve credit. You are very lucky to have home schooling, but sometimes it does reach a time where it may seem isolating or daunting because of the intensity of the revision in the expected study time. You have the maturity to know that your own emotional and physical health can be key to having a sharp mind. Many people never acknowledge this in a lifetime. Life is about negotiations, and knowing when you need to tap out is one for real. You may find amending the schedule by amending the bursts of mental pain actually alleviate some of the dread of and/or mental exhaustion. You might find it a pain in the backside to keep doing revision and learning but knowing the pain will eventually end pretty soon could be enough to help you cope.
Keep talking with your Mum who I am sure is very proud of you and has your back - Revision is not eternal and it has an end point.
Uni work never really gets a great deal harder than A-levels - there is just more of it and in a quantity that is spread over many more months. This is manageable (that is not a threat) You are going to be just fine.
I think you are very astute and very wise - 'Try it' is the only thing I can say. You are also careful enough to discuss this with your Mum and I hope you find the best balance for you that enables you to do as well as you can ever hope for.
Do get a balance between the hours of absolute inactivity with refresh of physical activity. Go walking, running or workout and don't forget to find time for laughter - with family, friends or just anyone who is a happy soul. When you get disillusioned - one mark can be the difference between and A or B grade. Hundreds will give up on that same journey (your gain) and that one extra mark you can gain can be the difference. If you have a bad day just learn one more small thing - just one more thing than you knew yesterday.
Good luck