We want to follow Nishkam-Karma (post#19), but we find it difficult.. 
We can to follow a healthy diet, but we find it difficult.. 
We want to go to gym everyday, but find it difficult.. 

Everything we know is good for us and want to do, but we find it difficult to pursue.. Why is that? 

May be, if we know how our decision making process works, we might be able to control it better.

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We all know, we have physical body with various sensory organs, hands and legs. But, body is not the boss of our system, right.. It’s mind (mana) which controls the body. Mind tells what to do and what not to do, and enjoys the pleasures through various sensory organs. 

But is Mind (mana) the real boss here?

For example, if a toddler goes to ice cream shop, he/she goes crazy over wanting stuff, but adults don’t do that, right.. I am sure, many of us, in our mind go crazy, but we don’t let that out.

Why is that, because over the period of time, we have learned, what to do and what not, how to behave etc. In short, it’s our intellect/Buddhi, which controls the mind.

But, is Buddhi the real boss?

Many a times, we take decisions, which can’t be explained by mind/intellect. They just come to us, and we feel it’s right to follow them, and they overrule Buddhi/Mana. People call it by different name like “internal-voice”, etc. 

In Vedic terminology it’s call as Aatma. It is the real YOU inside YOU. Finding that real one, is goal of all Vedic knowledge. It’s the real boss of our system.

Aatma controls Buddhi, Buddhi uses mana to control the physical body (Indriya) to get the life going.

A very beautiful description of this is given in Katha-Upanishad (1.3.3-4):

आत्मान्ँ रथिनं विद्धि शरीर्ँ रथमेव तु । बुद्धिं तु सारथिं विद्धि मनः प्रग्रहमेव च ।। 
इन्द्रियाणि हयानाहुर्विषया्ँ स्तेषु गोचरान् । आत्मेन्द्रियमनोयुक्तं भोक्तेत्याहुर्मनीषिणः ।।

Atman is the master of the chariot, body is chariot, intellect is the charioteer, and the mind are the reins, sensory organs are the horses. 

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Coming back to original question, why do we find it difficult to follow what we decide? 

Because we let charioteer, reins or horses to control the show.

We have to learn to use the master power we have to control the direction/destination of our lives. Use that power to overrule, Buddhi/mind/indriyas, and you will find things becoming easier..

All the Best!