Step one is to get better at the fundamentals of throwing.
Here's an analogy. You play the guitar. In the comfort of your own room you play pretty fine, but as soon as somebody asks you to play for them, you get your fingers crossed up and what you thought was easy becomes a lot more challenging.
Why? When you're on your own, there's less pressure--you can get by dedicating relatively less attention to what you're doing...but as soon as you have some extra pressure, you have less attention to work with (because this other person/thoughts of them are occupying your mind somewhat), and what used to be comfortable now needs your full, undivided attention just to come close.
You can deal with this a couple ways--learning to get more comfortable under pressure is one way (marker drill, more game experience, etc).
I'd say that you'd probably do with some more focused work on your throwing though--you're making mistakes and losing accuracy. These things have roots in your fundamentals--maybe you rush under pressure, or because you get tense you're using your arm to throw instead of your body. You can cull out these root elements in pressure situations; marker drill is great for that.
However, once you realize what needs work, you need to take the time and do some dedicated practice to get it right. Don't ever JUST throw. Picture a mark up against you, and take a second to focus, take a good step and make a good throw. Build slowly from a point of being relatively unconfident even at half speed to a point where you don't have to think about it anymore--and then keep working to speed up those same fundamentals (for me: step out and be set before I throw, as well as grip the disc) until you reach game speed.
I wrote a lot more about getting to a game-ready state on my blog:
http://mmackey.blogspot.com/2008/02/stop-thinking.htmlIt takes some dedicated work, but it's doable.