It has become very difficult to shift into 1st gear on my 1995 VW Golf. Pushing the shifter fully left and then forward now puts the transmission into 3rd.
How can I fix this?
As Flimzy's comment suggests, many transverse-engined FWD cars, including Volkswagens, use a series of ball-joints with plastic bushes in their gear linkages, and these can fail over time.
I expect you will find two sets of linkages - one for the side-to-side movement of the gearstick, and one for the fore-and-aft. I suspect it will be the former that has failed - do you get fifth when you try to engage third and fourth instead of second?
This is a common failnig on Peugeots from the late 80s and early 90s, we used to fix it by looping zip-ties around each bar of the linkage just before the ball-joint to tie them together and stop the ball-joints popping apart.
Another, less likely, possibility is a failed gearbox mount. This would cause the gearbox to drop slightly and thus throw the linkages out of alignment. I suspect you'd see other symptoms if that was the case however!