Has the problem always been there or has it just got worse.
Does the "gate" feel like its in the correct place, or like it says on the link, does it feel like its trying to go to 3rd? That would seem to be a selector issue.
If the car is just struggling to get the gear (can you get 1st and 2nd with the engine off) with the engine running, it could be the clutch....
If its the clutch it could be the clutch itself, or it could be the cable needs adjusting or will require bleeding if its hydralic.
Very hard to diagnose without seeing the car...