I like the other answers, so read those first, but I wanted to throw more ideas your way.
I really understand getting frustrated when attempting a technique, new or old. It's the same frustration when your fingers hit a wrong note when playing an instrument. When it seems like all you can do is play wrong notes, who would want to continue!?
In that sort of situation, don't try to NOT be frustrated, because if you can't 'un-frustrate' yourself, you'll get MORE frustrated! See instead if you can channel that frustration into slow, total perfection (of course you may not achieve it, but the point is NOT to achieve perfection, but rather to get as close as possible in that moment). Concentrate on a single aspect of each movement individually - while accepting that certain aspects may not be of the quality you desire.
There are a few specific aspects of a technique you can concentrate on at once, before you put it all together without thought.
First is balance - throughout the technique, go very very slowly, and hold your balance as perfectly as possible. There is a walking style called Namba Aruki that should help you with this - one of the things they never teach you in Aikido (and other martial arts) is that the walking style is completely different than standard Western walking. It requires a lot more resting tension, but a lot less energy overall, since you don't have to catch yourself for each step. It's a lot easier on your knees, too, though ease into it at first.
Second is effort - often we'll hear that Aikido should be effortless. When doing techniques slowly, though, it's far easier to exert force and use muscles. Focus totally on reducing the amount of energy you're putting into the technique. Think about aspects such as levers and fulcrums and how they work in Aikido. Aikido (and a lot of martial arts) is ALL about levers, fulcrums, and using those principles to your advantage.
Third is Extension - this one is more interesting, because it involves tricking yourself. Remember that each move in Aikido is either Forward (Irimi), Backward (Tenshin), or Pivot (Tenkan). It's doubtful that you'll reach the point of perfection, only performing those three movements (the closest I've seen is a 7th Dan from Japan - all he did was in, out, and spin - literally nothing else, and people went FLYING). When doing a technique, a lot of people will be watching themselves internally and thinking, "ok, hand goes here, then pivot, then step". Instead, break it down into those three movements (in, out, pivot) and then when you have to do those movements, visualize that you can move a mile in each step - so you aren't just stepping forward, you're stepping through. And for pivot movements, visualize a cylinder with a belt around it - you move into the pivot point, holding perfect balance, pivot around an imaginary vertical that runs from the center of the earth to the tip of the sky, and then move on. But the pivot, the rotation movement, doesn't exist outside that pivot point.
I hope these ideas can help you with your practice, but no matter what, hard work and dedication will be what propels you to the next level. But remember, there will always be a new level to achieve, always something to improve.
Have fun!
