Restoring self confidence - possible?
I had some similar experiences in the recent past.  


  I still feel I am far away of being an expert in it


No one can be an expert in a whole domain. Being expert/perfect is an ideal situation in my personal opinion. So in case you are not an expert, there is nothing bad in it. You admit that you are not an expert so this is a positive point, it means you are able and willing to improve yourself and your knowledge. It means that even if you get a PhD degree, you'll still try to get more knowledge. 


  even if I do get my PhD I think I will feel like I don't deserve it


You seem to be a perfectionist. But remember no one is actually perfect. Each one of us has some strengths as well as weaknesses. How can you measure perfection? This is not absolute in my personal opinion. Have you ever thought why relative grading system is mostly used for evaluation instead of absolute grading? Because we cannot live in isolation. We are living in a society,interacting with our fellow human beings. So we need to be evaluated relative to our fellows. If you don't deserve a PhD, what about your fellows? When your supervisor is sure that you have possess certain knowledge level at which you can present your work, it means it is relatively better than many others. If those others can get a PhD, you can also deserve it. (Even if you ACTUALLY don't deserve a PhD after some time when you'll get out of this phase of life, you'll start thinking this way).  


  In retrospect, all the proofs I stuggled seem so easy to me


Why don't you try to think yourself as an intelligent or genius person? The same proofs may be too difficult for someone else who is not actually interested in this domain. You may disagree but in my personal opinion, apart from the industry-academia discussion, you are interested in your research domain because without interest one can't achieve this level. However, you have been working too much on the same things for a long time that you have got bored from the same lifestyle and you need a break and a change. 


  Although I never considered acedemia as a career path and only consider industry


I consider industry-academia debate to be separate from the research-development debate. And i think you are getting confused between the two. Industry is incomplete without academia and vice versa. Similarly research and development support each other. Most of us consider one of them to be more important that the other and hence get confused. We can characterize them in two ways... 1. what we get, 2. what we give
 Try to think seriously about choosing industry, academia or a combination of both and evaluate the pros-cons of each choice in terms of your time, energy, monetary benefits, work-life balance many other job motivational/demotivational factors. However, according to my personal experience and understanding, you are sick and tired of your routine in academia and you have been overloaded with work in the recent past which has disturbed your work-life balance. Considering industry as an alternative would only worsen the situation because job in industry can be more time and energy consuming.  


  I work 10 hours each day, at most a day off weekends and having no time left to cultivate my interests, develop new friendships or develop other useful skills.


Take out some time for yourself. If you are able to afford, take some leave from your current job (atleast a month or so) and spend some time at home with your family and friends. Make sure that you are completely disconnected with the official/research work/supervisor and any relevant person who can give you relevant news. After spending around 15 days of your holiday, start working part time with some other faculty member of almost the same domain but with relatively different research interests as your supervisor. You'll probably automatically start comparing that faculty member with your supervisor and will most probably start missing your supervisor. You'll realize that you were taking him/her for-granted. Its just that you have been working with your supervisor for some time now and you know much about him/her technically so you don't find something new in this experience. As a result, you start thinking that their is nothing special in what you are doing. Hopefully at the end of your holidays,you'll be back fully energetic and enthusiastic to pursue your work.