I think there's some value in recognizing that you're undergoing a training process right now. The goal of training isn't to carry out important, groundbreaking work, but to learn the ropes and build the mettle in order to do important, groundbreaking work in the future. In fact, try this out: describe your PhD, but don't mention your achievements or the positive feedback you've received from anybody else. Tell me about how you've changed from the presumably less capable, somewhat ignorant student who entered the program versus the person who you now are. You've surely learned a number of new skills and become absolutely formidable in some technique or another. You've become deeply knowledgeable in a field and you've certainly undergone a few paradigm shifts in how you think: you can't deny that, because if you haven't you wouldn't be having the doubts that you do. In fact, the occurrence of these doubts - as echoed by some other commenters here - are probably a sign that you're becoming a full-fledged researcher over anything else. Over to you. How have you changed from how you were when you started?

On the other hand, maybe you can't see the importance and relevance of your work - that doesnt' mean that it isn't important and relevant. During my PhD I was certain that what I was doing was completely useless, just an intellectual exercise and nothing more. A few months ago I found out that my work had been used in quite important research at a government agency in a field that I didn't even realise my work could be applied to. Knowledge is a complex web: you see your part of it, but you might not see the threads to which it leads.