Yes that's true! I have the PC version and I haven't even reached 70% of the game! I don't know why but I think it's because every time I fire up the game and say "Okay, today I'm gonna finish the main missions and the story" but 2 hours later I was flying cars and blowing up cops left and right.
I know how it is, really. I was completely mesmerized by the visual abilities it had to offer back then, and I'd have simply installed a mod or two to compensate with the fact that some missions were simply too hard for me to complete, like the desert airplane mission where you had to go through all the red circles, then return before you would've run out of fuel. I think I stayed, without even joking, a "mere" 2 days before actually feeling the course of the mission and, ultimately, achieve its milestone. Oftentimes, when only triads missions were on the map, I'd simply wandered in the desert, searching for easter eggs ( I myself have managed to find the one with the supposed human bones), or spawn a jetplane to fly above the area 69 (silly name) to fight with the automated rockets. Good times, back then. I do hope you finish it, you will be completely astonished by the ending of it, I can guarantee you.
As for me the pc was reformatted and the installer got lost, so I lost the GTA SA game. I did download another installer but I haven't installed it yet in the pc since I don't have much free time to play games unlike before.
I feel your pain it is the same thing that always happens to me when I am playing a GTA game. Its like I start out with the intentions to play some of the missions in the game and then I end up doing all sorts of other stuff. The game just has too much to do. So what I do now is when I turn on the game I just head straight to the mission points and do my best to just do the missions and try not to get sidetracked because it is so easy to do that.