Transmission Not Go Into Gear problems of the 2009 BMW M3

I was driving on a two-lane street (one-lane each direction) and as I was approaching a stoplight I turned on my left turn signal and started to move into the dedicated left-hand turn lane. As I did this I needed to downshift as I slowed down for the stopped traffic ahead at the light. When I shifted to second gear and pressed the accelerator to move into the turn lane my car would not engage into gear. I was almost rear-ended from the cars behind me but luckily my car's transmission engaged at the last second and I cleared the lane I was in. But because I was pressing the accelerator so hard to try to get the clutch to engage, when it finally did I shot off across the left-hand turn lane into the lane of on-coming traffic. I was able to steer clear before being hit head-on by on-coming traffic but the experience left me very uncomfortable with the transmission in my vehicle.