Eternity is not the continuity of time forever. That is the meaning in the dictionaries: forever and forever. But forever is part of time--prolonged time, indefinitely prolonged, but it is still time. Eternity is jumping out of time; it is nontemporal, it is no-time.
The present moment is the door to eternity. The past and future are part of time. The present is not part of time--the present is just between the two, between the past and the future. If you are absolutely alert, only then are you in the present; otherwise you keep missing it. If you are not alert, by the time you are alert it is already gone, it has become the past; it is so swift.
So between the past and the future there is a door, a gap, an interval--now--that is the door to eternity. Only in eternity is bliss possible: in time, at the most, pleasure; at worst, pain--but both are fleeting. Their nature is not different. Pain comes and goes, pleasure comes and goes. They are momentary, water bubbles.
Bliss has no counterpart. It is not a duality of pleasure and pain, day and night. It is nondual, it knows no opposite. It is a transcendence. Try to be more and more in the present. Don't move too much in imagination and memory. Whenever you find yourself wandering into memory, into imagination, bring yourself back to the present, to what you are doing, to where you are, to who you are. Pull yourself back again and again to the present. Buddha has called it recollecting oneself; in that recollection by and by you will understand what eternity is.