Golden Rule of flying a Cutter: NEVER boost towards a mail slot. When you arrive next to a station, just use regular thrust.
I always boost when I exit SC on my way to the station. Of course, before dropping from SC, I make sure that when I drop, I will be directly facing the slot. Then, immediately after boosting, I apply full reverse thrusters. I can always slow down under the speed limit before getting to the slot.