I drive a 03 g35 6mt coupe & lately whenever I'm about to brake I can't downshift.My shifter locks up until I do a hard 1step & then it unlocks could somebody give me some advice on what causes this problem & how to fix it

 

Worn synchros and not properly rev-matching. I'm not intending to insult you but it sounds like you don't know how to properly drive a manual transmission which a LOT of people don't due to hollywood misinformation and preconceived notions about how it's supposed to be done.

You don't downshift at an idle rpm, you tap the throttle and apply "just the right pressure" to the shifter and as the rpm's are matched it just falls into gear. The trick is being ready for it and releasing the clutch while keeping rpm's steady at that EXACT moment, definitely takes a lot of practice.

Worn synchros, internal parts of the transmission, the blocking rings gets worn over time as they're constantly grinding against the dog clutches. Eventually they start to engage harshly, this is MASSIVELY amplified by having old gear oil. The thinner and less viscous the less torque is applied from the blocking ring into the dog clutch, they are no longer spinning at the same (nearly) rpm and won't mesh together until perfect. You can try a fluid change which will almost always help but sometimes when the transmission has significant wear you will need to use better quality fluid like Redline, I was using Redline MT-85 for an entire year before I swapped transmissions and you would have never known I had a synchro problem.