Gear shifting is too tight

Hi
I have a Pulsar DTSi 150 of 2004 which has clocked 43000.I love my bike and maintain it well.For the past 2months I started feeling so difficult to shift the gear.I changed the oil and but of no use.Cluth lever is not so tight but gear dosent seems to fall,so I have to kick the gear pedal.Some time it even gets stuck in some position.
wile lowering the gear from 5th to 4th I feel loss of power even at the prescrided rpm,and the veichele starts to knock the engine. so i have to lower to 3rd gear.like wise if its in 4th gear have to shift down two gears to gain power.
Pushing the veichle in 1st gear holding the clutch is so difficult.I tried to do the same with my friends pulsar and feel my veichle is so difficult to move.

Kindly help in this regard