How to cope with feelings of powerlessness on a PhD?

I am currently a third year PhD student of pure mathematics.

I've been dealing with a feeling of burn out for a long time now. Quite frankly I struggle to find interest in anything related to my branch of science right now and am all but fully decided not to pursue academic career afterwards anyway. This furthers the feeling of disinterest in my field as anything I do seems irrelevant to me in the future.

Furthermore, I have a major personality clash with my supervisor in that I would like to approach PhD as a regular job, establishing a routine et caetera, while he is a very spontaneous person, often coming up with a brilliant idea in the middle of the night and wanting to share it ASAP. Also I am sure that if I told him I want to abandon academia he would be quite disappointed in me (to put it mildly).

Because of that I find it very hard to motivate myself to read papers or even textbooks on my chosen subject. The feeling of falling behind furthers the depression and inability to progress in anything. While everyone talks about how much new skills and knowledge they have mastered throughout their PhD, I feel as if I haven't learned almost anything new since I finished my Masters. I look at my PhD and don't know what I can do to make it better and I am very unsatisfied with how it is going so far. I thought that maybe trying to just write up everything I have so far would rekindle the motivation in that I would at least see I already have 20/50/100/whatever pages worth of work, but just thinking of writing everything up is so intimidating I am unable to even start doing it.

I feel like a terrible student and a horrible person in that I am wasting so much time I am, in principle, getting paid for (from my funding). What can I do to get as much out of my PhD as possible and to finish it to satisfaction of all interested parties?