I have the same problem and I know what is wrong (doesn't do ya much good though):
When you miss you're shift into 3rd gear, 99% of the time you're throwing it in between the 3rd and 5th gear, which will eventually wear out your 5th gear synchro. When you miss the gear you're reverse lockout comes on because it thinks your 5th gear synchro just went out and it locks out reverse cause when your 5th gear synchro is out, like mine is, it slams out of gear and would go into reverse on the highway if the lockout wasn't activated.
On cars whose 5th gear synchro is fine, it should go into reverse after a little bit of driving. If it doesn't go into reverse, you CAN force it in, but it's not good on the tranny. If it doesn't go in, you've got to:
Clutch in, pull the shifter back into reverse, it wont go in, but just pull it back, let the clutch out SLOW, it'll grind and pop in, if it doesn't the first time and grinds and pops back into nuetral, you've got to pull back harder. After a few times of doing that it will go into reverse. Drive in reverse for a little while. Then it will probably get stuck in reverse and you'll probably freak, but don't worry. Now you've got to do the same thing in the opposite order. Clutch in, push up on the shifter, let the clutch out slow, it'll grind and pop out, then you should be able to move back and forth into reverse but it will be really hard to get in and out the first 5-6 times but then it will be fine.
I abosolutly do not recommend doing this. But I had to do it today and am going to have to do it again tomorrow.
Good luck.