How does one keep oneself motivated to follow the path of dhamma? As a lay Buddhist, given that it is terribly difficult to attain to Nimmata or the Jhana and at point when Nirvana seems to be just a wishful thinking able to be attained by superhuman ability and exertion, how does one motivate oneself to continue?
Every single meditation seems like a postdated cheque in a non-existent bank, how to keep going?
I come from a non-Buddhist background and ask myself why should one continue if the path itself is full of suffering?
When I go to the doctor and he does a blood pressure reading, sometimes I am anxious about how the reading will turn out. This anxiety in turn makes the reading go higher, which is not what I wanted in the first place. To get a good blood pressure reading within a good range, I need to let go of my anxiety and expectation.
It's pretty much the same with regards to doubt and your practice.
More on doubt below ...

There are the five hindrances which can prevent you from progress in your practice. They are: sensory desires, ill will, sloth and torpor, restlessness and remorse, and doubt.
What you're experiencing here is doubt - &quot;can I do this?&quot;, &quot;is it possible?&quot;, &quot;where am I going with this?&quot;, &quot;is this the right way?&quot;.
The general advice for overcoming the hindrance of doubt is letting go and trusting the inner silence (see quote below).
Here's Ven. Ajahn Brahm's advice on overcoming the hindrance of doubt, from this essay:

Doubt refers to the disturbing inner questions at a time when one
should be silently moving deeper. Doubt can question one's own ability
&quot;Can I do This?&quot;, or question the method &quot;Is this the right way?&quot;, or
even question the meaning &quot;What is this?&quot;. It should be remembered
that such questions are obstacles to meditation because they are asked
at the wrong time and thus become an intrusion, obscuring one's
clarity.
The Lord Buddha likened doubt to being lost in a desert, not
recognising any landmarks.
Such doubt is overcome by gathering clear instructions, having a good
map, so that one can recognise the subtle landmarks in the unfamiliar
territory of deep meditation and so know which way to go. Doubt in
one's ability is overcome by nurturing self confidence with a good
teacher. A meditation teacher is like a coach who convinces the sports
team that they can succeed. The Lord Buddha stated that one can, one
will, reach Jhana and Enlightenment if one carefully and patiently
follows the instructions. The only uncertainty is 'when'! Experience
also overcomes doubt about one's ability and also doubt whether this
is the right path. As one realised for oneself the beautiful stages of
the path, one discovers that one is indeed capable of the very
highest, and that this is the path that leads one there.
The doubt that takes the form of constant assessing &quot;Is this Jhana?&quot;
&quot;How am I going?&quot;, is overcome by realising that such questions are
best left to the end, to the final couple of minutes of the
meditation. A jury only makes its judgement at the end of the trial,
when all the evidence has been presented. Similarly, a skilful
meditator pursues a silent gathering of evidence, reviewing it only at
the end to uncover its meaning.
The end of doubt, in meditation, is described by a mind which has full
trust in the silence, and so doesn't interfere with any inner speech.
Like having a good chauffeur, one sits silently on the journey out of
trust in the driver.

Also see his conclusion:

Any problem which arises in meditation will be one of these Five
Hindrances, or a combination. So, if one experiences any difficulty,
use the scheme of the Five Hindrances as a 'check list' to identify
the main problem. Then you will know the appropriate remedy, apply it
carefully, and go beyond the obstacle into deeper meditation.
When the Five Hindrances are fully overcome, there is no barrier
between the meditator and the bliss of Jhana. Therefore, the certain
test that these Five Hindrances are really overcome is the ability to
access Jhana.


Ven. Ajaan Fuang Jotiko gave a shorter version of the solution to overcoming the hindrance of doubt in  &quot;Awareness Itself&quot;:

A student came to complain to Ajaan Fuang that she had been meditating
for years, and still hadn't gotten anything out of it. His immediate
response: &quot;You don't meditate to 'get' anything. You meditate to let
go.&quot;

The answer to your inner doubts is ... just let it go!
Does it matter whether you gain Nirvana is 10 months or 10 years or 10 lifetimes? Is there a deadline?
Just keep practising with heedfulness, as the Buddha stated in the Mahaparinibbana Sutta:

Then the Gracious One addressed the monks, saying: “Come now, monks,
for I tell you all conditioned things are subject to decay, strive on
with heedfulness!” These were the last words of the Realised One.