My PhD seems daunting and overbearing. How shall I regain motivation and interest? EDIT: I should clarify this is not the first draft of my PhD, it's the first draft of my upgrade document to go from MPhil to PhD.

I have been working on my PhD for about two years now, it is a six year part-time program. I have a very good relationship with all my supervisors and they are very supportive. I am fortunate to be part of a good university which is open-minded and free-thinking. I also have attracted the interest of a well-known external academic who is an authority within the field and who wants to be involved. I am very lucky.

The problem is that I am just not that interested or convinced by my research [chiefly its significance]. I have to submit my upgrade document [MPhil > PhD] in about five week's time, and the viva is a week after that. I have written a draft document which is 12,000 words and my main supervisor has written a lot of comments which are suggestions for improvements. The issue is that I do not feel that interested or compelled to work through them. 

I am under the impression that my supervisor is not that convinced by my research, even though she keeps saying she has no concerns. I feel my research is too vague, too broad but also too overbearing. It incorporates three disciplines [at the advice of one of my supervisors], disciplines A B and C. Discipline A I am not that interested in - I was for my master's but now I find it quite tedious but it is quite an important part of the whole. It is a well-worn discipline and feels rather old and stale to me now. Discipline B is quite new and interesting, I have published within this discipline. Disipline C is very new and up-and-coming. I find it quite exciting and my supervisor has written the least amount of comments about it in my draft paper, which I take to mean she has the least amount of complaints about it [she has a lot of comments to make about dispclines A/B]. She has also said that discpline C is a 'key' part of the PhD. 

I do not know what to do. I am really not that interested and the whole thing seems so daunting. My PhD focuses on A and B mostly with C being only a feature, but I am wondering whether I should switch things round entirely and make it more about C. I feel I will not really be able to convince my panel about the significance of my research in the viva if I am not excited about it or convinced myself. In my master's I was always keen to research and write, now I am not, but I know I would be if the topic were carried out a bit differently. 

What shall I do, how shall I regain interest? My PhD seems rather fluffy and not sharp enough. Additionally, I am daunted by the scope of the whole thing: it seems too large to work through and 'bigger' than me. I am kind of at a loss for what to do.
my supervisor (...) keeps saying she has no concerns


Believe her.

Having lots of comments and suggestions for improvement does not mean she doesn't like what you have done. It may be the exact opposite: because she likes what you have done, she wants to make sure your research is presented in the best possible way, clearly demonstrating its potential and value.

I would suggest taking some time off to recharge your batteries and then working through the list. You've made it this far, now do what it takes to complete your PhD. Judging the value of the research is most likely better done in a less stressful situation (i.e. once your done).