This could be a nearly free thing to fix, and it could be a new tranny needed. I'd start by checking the linkage to make sure that stupid dowel pin is in there tight, and that nothing on the linkage is loose or obstructed. Something could just be in the way. Then I'd re-adjust your clutch master cylinder so that there's at least 1 inch of free play in the pedal. It may be pre-loading your clutch (keeping pressure on the fork), keeping it from fully engaging. This will make you pop out of gear.

The worst thing it could be is a busted shift fork. It's like a $50 part inside your tranny, but labor to replace it is so high that you'd be better off replacing the tranny. I hope that's not your deal.

I broke my 1/2 gear shift fork in my non-civic. I have to rebuild it because a new unmodified AWD gearbox is $3000. Mines pretty heavily modded. Dang.

Good luck with your WRX, and welcome to turbo-AWD world!!! :thumbup: Now don't go breakin' that tranny, too. It costs a "tad bit" more than a Honda box to fix.