Supervisor appears to have a different scientific attitude I'm a first year PhD student in the life sciences. So far, my PhD has been going well. I meet my supervisor regularly and he is interested in the project and generally supportive.

A couple of months ago my supervisor encouraged me to start writing a draft for a paper, incorporating the results I have so far. We regularly meet to discuss parts of the draft, mainly figures and main statements. 

The problem I noticed once we started working on the draft is that my supervisor has a different attitude towards the project. Very often I try to emphasize that I am uncertain about a statement, a method or whatever but I feel unheard and ignored. I feel as if my doubts are not taken into account when we work on the draft at all. It appears as if my supervisor has this grand idea already fleshed out in his head and appears unwilling to hear that some statements might have to be toned down a bit. I feel uncomfortable and depressed as of late, as it is me who is ultimately responsible for producing a piece of scientific work that adheres to the standards of scientific rigor that I think are correct. I feel pushed into doing something that is wrong.

Could it be that I overthink this? Are there different shades of scientific rigor? 'Selling your story' is part of the modern scientific world, right? Do young PhD students tend to be overly defensive and critical of their own work? Is this just 'how the game works'? Should I just trust my supervisor?
I think you may be attempting perfection (though you may
 not define it as such), but the reality of research is that it is constantly evolving, and may never mature into perfection.

Conflict, rejection and skepticism are all indeed 'part of the game'. If not your guide, then an editor, a reviewer or a fellow-researcher will (almost certainly) at some point refute your ideas. Many times they will not (and may not need to) justify themselves, which will add to the discomfort. 

If an idea is wrong, it will be pointed out at some stage, maybe before submission, or during review (and sometimes after publication). Even if this happens, it will not blot your career as a researcher (unless the mistake is very fundamental, targets other researchers wrongly or involves some kind of trickery). So you should do your due diligence and rest assured after. By due diligence, I mean being thorough, verifying and re-verifying until you are satisfied that your work is technically and logically sound, in all aspects.

Also, never forget that the work is a partnership between you and your guide. Any responsibility for mistakes is held jointly between both. The more junior you are, the greater is the guide's responsibility.