Why is that the pleasent feelings last for small time while unplesent feelings seem to last forever?

Both pleasant and unpleasant last forever but we only remember unpleasant. Our craving never stops, if we get something that makes us happy our craving does not stop there it wants more pleasant, more and more..thats why the pleasant becomes unpleasant after sometimes..on the other hand we want to forget unpleasant and that makes us think more about unpleasant..

One who does neither grasp after pleasing, nor unpleasing, not neither-pleasing-nor-unpleasing feeling, such one has arived under the Khema, secure.

Seeing pleasant as an arrow, unpleasant as a pain, knowing neither-pleasant-nor-unpleasant will not remaind, the wise has abound to be touched by what ever feeling may arise.

Pleasant sensations are what the egoic mind expects. They do not present themselves as problems (they only become problems when they are absent), so the thinking mind does not focus on them or track them over time. Pleasant sensations are merely experienced.

Unpleasant sensations are viewed by the egoic self as intrusions — things contrary to what it expects — and thus as problems to be solved. The thinking mind focuses on them, tracks them over time, and analyses ways to eliminate them and gain (or return to) pleasant sensations. Unpleasant sensations are not merely experienced; they are attached to past, future, and the desire for a different condition.

In other words, the only difference between pleasant and unpleasant sensations is that the thinking mind engages the latter. The egoic self marks the time of unpleasant sensations as part of its efforts to rid itself of them. During pleasant experiences the thinking mind drifts off, having nothing much to do.

This is part of the practice of Vipassana meditation, incidentally. In Vipassana body-scans one seeks out aches and irritations and watches them, seeing them as mere (impermanent) sensations. This disrupts the fixations of the thinking mind, with the result that these unpleasant sensations come to have the same evanescent quality as pleasant experiences.

Pleasant feelings last as long as we enjoy them.

MN44:24.2: “Pleasant feeling is pleasant when it remains and painful when it perishes.

Painful feelings are pleasant when they end.

MN44:24.3: Painful feeling is painful when it remains and pleasant when it perishes.

Painful feeling do cease eventually.

DN34:1.4.27: Renunciation is the escape from sensual pleasures. The formless is the escape from form. Cessation is the escape from whatever is created, conditioned, and dependently originated.

However, painful feelings will persist as long as we insist on craving pleasant feelings.

MN44:25.2: “The underlying tendency for greed underlies pleasant feeling. The underlying tendency for repulsion underlies painful feeling. The underlying tendency for ignorance underlies neutral feeling.”

Struggling to escape from painful feelings by pursuing pleasant feeling perpetuates this endless cycle. The escape from this cycle is to relinquish the tendency to hold on tight:

MN143:6.1: You should train like this:
MN143:6.2: ‘I shall not grasp sight, and there shall be no consciousness of mine dependent on sight.’ …
MN143:6.5: ‘I shall not grasp sound …
MN143:6.6: smell …
MN143:6.7: taste …
MN143:6.8: touch …
MN143:6.9: thought, and there shall be no consciousness of mine dependent on thought.’

If painful feelings last longer than pleasant feeling, then there may be benefit in examining such continued existence.

MN38:17.8: Craving is a condition for grasping.
MN38:17.9: Grasping is a condition for continued existence.

Grasping at pleasant feelings, wishing them to continue creates suffering. Pleasant feelings always vanish and that vanishing is painful. It remains painful as long as we grasp at the memory of the pleasant feeling that is now gone. Wanting pleasant feelings to continue after they are gone is painful. So, by not grasping at the pleasant feelings, unpleasant feelings will disappear on their own.