Yes, this is a learning experience that all transitioning drivers have to go through. With rear brakes only (in my case, TAG), you can usually use the rear end much more to slide/rotate the kart on turn in, and trail braking is very common with rear brakes only.
On the shifter, the consensus seems to be that braking should be done hard and in a straight line – hence, we go into corners deeper that a TAG, also a much higher speed – then substantially or fully released when turning in. The power of the shifter also lends itself slight powersliding at apex through exit, but you have to be careful with this, as it can quickly turn into an exit spin (very rare for me on the TAG) and/or front tire oversteer wear.
Try to find the better shifter drivers at your track and follow them and/or talk to them about their lines – almost all have come up through TAG or HPV/KT.