i've got my piano grade seven in about a month and i've got a dilema about one of the pieces.
it's my b piece, the wagner lied ohne worte, and i absolutely love it. i've been playing it for almost a year, and unintentionally quite a bit under the suggested metronome marking. i've grown to love it at this speed, and my teacher's making me speed it up but i can't bare it at the suggested speed, i feel its lost all the beautiful calmness and gentle suspense which i had it at. my teacher doesn't normally insist that i stick to editorial suggestions but this time she really feels i need to, which makes me think i must be wrong. but when i play it now it doesn't feel right, i almost can't bare playing it that fast and i'm really struggling now to portray all the emotion that i have no problem with at a slower speed. i don't want to lose marks in the exam, but i want to play it MY way.
what should i do?

It's good to be guided by your teacher but there's your opinion should count too at this level. Professional recordings differ so much in tempo so your ideas are quite likely as valid as those of your teacher.
With only a month to go it sounds to me a little late to start making changes like this.
You do say that you'd unintentionally played it slower.... maybe you need to convince your teacher that it's a considered decision rather than an accident?
If you could find recordings at different speeds it may help to support your view.

Maybe you could meet somewhere in the middle. If you're playing it waaay off the suggested speed, it probably wouldn't go down too well in an exam. Also, things often seem a different speed to what they actually are when you're playing them, so have you tried recording yourself then listening to it back?

Try to understand your teacher's point of view and make sure you seriously consider it. If, however, you still aren't convinced by the suggested tempo then I'd say you should play it the way YOU think it should be played. If you're not thoroughly convinced by what you're playing then what chance have you got of convincing the examiner? It looks as though this is the tempo you've decided on through studying the music and this is how you want it to sound.

My initial instinct is that you should try and play the music as the composer intended it. However, it is not always easy to know what that is! Provided it is not wildly slower than the marking, you should stick to your guns, as you feel you can give a convincing performance at this speed.

Is the suggested metronome marking what Wagner originally wrote, or has it been inserted by the editor of the music you're playing from? It may be worth going to an Urtext edition to check. Then if it's what Wagner wrote, I'd be inclined to go with that tempo.

And to get to the faster tempo, I'd practice with a metronome over two weeks, starting at your tempo, and gradually increase the tempo over the two weeks to the suggested tempo. Hopefully that way, you'll manage to keep a lot of your personal interpretation, and get you comfortable with the new tempo.

Hope that helps. Best of luck!!