i manage to beat off the 4 tentacles then when the 5th tentacle takes me on the ride i spam the left mouse button but it drops me half way through the ride , n it happenes the same thing again n again ,, wht should i do ??

If you get dropped and the kayran starts throwing rocks, you did it right. (If you did it wrong, the kayran will throw you into the bridge).

Note that the collapsed bridge now looks like a ramp :)

The Witcher 2 forum has many threads like this. Some suggest that you should lower your resolution and graphics settings because some computers can't handle the graphics and the quick time events simultaneously. The "tap till you drop" QTEs seem to be bugged (judging by the number of "QTEs ruin the game" threads), and if nothing else works, you might want to lower the difficulty to Easy temporarily and disable QTEs (I've never done that so I don't know how, but they suggest it repeatedly in that Witcher 2 forum).

I not fully certain if thats the problem but I wonder if you've seen the QTE space prompt that comes while riding the tentacle. I think it works out that if you don't fill the bar you're thrown at the bridge and die. If you fill the bar you're dropped of safely.(then ride again)
At some time a space prompt will come and a scene follows where the bridge falls on the Kayren. It's a split-second prompt so it's easy to miss. Not sure if there are further conditions for the prompt to show up (fill the bar, ride certain number of times)