I am just starting out on my PHD in literature and I have already discovered that pretty much every time I get an email from my supervisor it feels like every idea I've had and everything I've done thus far just isn't good enough and that pretty much everything I am doing is wrong. Do/have other people experienced this phenomena ? And how do you pick yourselves back up after yet another disheartening email?

I think a lot of academic emails are to the point and short on niceties. I would try to see it as a positive that your supervisor is engaging with your work and suggesting improvements - if they thought it was hopeless, they wouldn't do that.

I would try not to take it so personally because they probably don't realise the effect it has or you. Your other option is to address it directly and let them know how it makes you feel. I think this is very usual though - your supervisors are supposed to correct your work/give you alternative ideas.

Try not to let it get to you. These supervisors are very experienced (generally!) and have a good working knowledge of what it is your thesis needs. They help you shape the project and learn how to do so.

Academics don't generally do the compliment sandwich. They'll get straight to the point, words are time and it's just easier to write out what needs changing. At a PhD level, it is expected that you can take their suggestions and incorporate them at a detached level, not feel disheartened (though a difficult feat in itself!).

Is your stuff good enough? No. It won't be for at least three years (every PhD goes through that), there will be multiple revisions, changes, more revisions. They'll want one thing at one meeting and change their mind at the next (as one of my supervisors used to say "forget what I said last meeting"). Prepare yourself for this, this is what academia is like. Getting comments back on journal articles will be worse. The amount of revisions you might have to do to just get an article published can be astronomical.

You are a PhD, which means you are still a student, you are still learning. You will be kept at a much higher standard than an undergraduate or a masters. They aren't trying to bring you down, they are trying to help you become a better critical thinker and a better writer, and the best way to do that is to push/challenge you.