Beautiful. In that case, it's all about your psychology! Really, it's all about manipulating yourself into being awesome.
So, in all likelihood, your peers perceive you as smart. Either you're in a bad school and smart relative to your peers, or you're in a good school and your peers automatically assume everyone else is smart. This is overwhelmingly in your advantage, because you KNOW that people
see what they want to see. Let's say you stand still because you're too nervous to move. They see someone who is bold enough to stand still and who isn't moving about in awkward little shuffles. You pause for a moment because you're afraid you might be talking way too fast. They see someone controlled enough to pause. Between their expectation and their physical distance from you, they won't even pick up on your shakes. There's two things at play here: perception and reality. Only you are aware of the complete reality. No one can hear or feel your heartbeat. They only get to perceive what you give them, and invariably they will look upon what you give them much more favourably that what YOU'RE looking at (because they don't have all the information). So long as you give a half-decent speech, it'll come off well. It's like meeting a stranger. When's the last time a friend of yours went "hey let me introduce you to a friend of mine!" and you automatically assumed they were going to be downright rotten and horrible. Probably never - it's just not natural to assume that. Same with your speech. No one assumes it's bad. They're already predisposed to like your speech, you just have to talk loud enough for them to hear, really.
Second point is very related to the first point but deserves it's own little bit. Fake it til you make it. Everyone gets nervous during speeches. From Year 9 - 11 I didn't even write a speech, I just rocked up on the day and said some words, and invariably I had everyone go "mad speech". In Year 12, I was STILL nervous, even though I knew 90% of the room agreed with me (pro gay marriage speech), and 90% like the way I give speeches. Still shitting myself. But I guarantee you not a single person in that room thought I was nervous. I would bet my legs on it. Why? I stick my chest up, my voice is loud, I pause appropriately, speak well. I don't do those things because I'm confident. I do those things because I am SO SURE that those things will be perceived as confident. It's like making a joke that you don't personally find funny, but you make it anyway because you know the person you're telling to will find it funny. Act confident, and have faith that your confident body language will be perceived as "confident guy" instead of "guy shitting himself and trying to seem confident". If you fake it confidently enough (

), then you'll always be perceived as the earlier.
What to do?
Have good posture - squeeze your shoulder blades. Look up. Either stand still, or move well and powerfully (don't shuffle five steps here and there). Loud voice with variation in tone. Appropriate pauses.
As far as your breathing... just use your pauses well. If you ask a rhetorical question for example - perfect time for a breath. An ellipsis, easy breath. Basically any mark of punctuation is time for a breath. Breathing is also a good way of making sure you don't run your words too much. Take breaths, and you won't come offlikeyouretalkinglikethisanditsagigantichashtagandnoonecanunderstandwhathefuckyouresaying. Funnily, the only way you'll run out of breath is if you talk like that.
Not going blank is never a guarantee. Have cue cards that are highlighted. Eg, if you have three sentences on a cue card, highlight each of them a different colour so you can differentiate between them (especially if they're cramped). If you go without cue cards, you're a beast, and you don't need to worry! Nah maybe worry a little bit -- if you go blank, just make it up on the spot. (Or take a dignified pause, try to figure out where you were, then start again.) I actually forgot half a monologue in my Year 12 Theatre Studies production. It was hilarious. Anyway -- if you forget where you are, you should almost definitely have a gist of an idea. What was your point again? Get that, give the basics of the point, then move on to the bit you remember.
Of course, remember that your aim is to convince your audience. Your audience is the most important part. Logic is great, credibility is great, but it doesn't mean shit if your audience isn't listening. Remember to talk like you want them to listen, and not to talk like you want to get it over with.
Yeah that's all I got. Good luck.