Something seems off. I’d make an appointment with a doctor, if nothing else just to rule out something medical. 
You have posted that you are:
and
Regardless of failing the test this is extremely worrying to me, there are many factors that can contribute to fatigue but you are showing many many signs of needing some good solid rest.
Take it extremely easy for at least a week and preferably 2, like ride in z1 for a bit easy and make an appointment with a doctor if possible to rule anything else out.
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You should not have felt “exhausted” throughout the test. Others have said this also. If you are exhausted climbing stairs, I would schedule an appointment with your doctor. Take 3-5 days off the bike, then ride z1/z2 for several days before looking at regular structured training. Being exhausted climbing stairs is more worrisome for me, possibly indicating a medical condition.
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Speaking from experience I can say that’s is worrisome, especially if its a regular thing which it was for me 
Thankfully that behind me now 
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I think rest up for at least a week and if you feel the same then head to the doctors… to be honest it might already be time for the doctors.
This is beyond the normal “what is real FTP and is it important?” guff…
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How’s it going now OP?
What equipment are you using? Someone here might be able to help calibrate your setup. For example, I previously had a wheel-on trainer (kickr snap) that was very sensitive to setup.
Sounds like you’re very fatigued. My definition of a rest, in these circumstances, would be a week of no cycling, perhaps a sports massage, and A LOT of protein. I find even active recovery rides are unhelpful if I’ve totally burnt myself. The body needs time off on those occasions.
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