How can we improve and strengthen our motivation?

So much depends on motivation; but can it be improved?  

Yes, I think so - but this is not easy or straightforward, and needs to be approached indirectly. 

(I mean, we cannot - because it does not work, and anyway is not what is required - simply decide on what is better, and then do-it by sheer will-power. A process of discovery is entailed.) 

First; we need to stop trying to fulfil somebody-else's agenda. 

This is, of course, merely negative and permissive: we then need to replace external motivation with an internal motivation that is both good, and sufficiently powerful to make a difference.

Secondly, therefore, we must discover our inner agenda - but that inner discernment needs to focus upon our eternal agenda. 

By "eternal" I particularly mean our post-mortal agenda: i.e. what do we want to happen to us on a timescale going beyond death*: an open-ended timescale.  

And this eternal agenda, in turn, depends on what we most value. 

We need, therefore, to discover/ decide what is is that we most want. 

For example (others will disagree); I personally most-value love, and from that creativity. In other words; what I most value and want on an open-ended basis, would be to live in a context of permanent inter-personal love, and to be active in creative (which includes procreative) ways. 

For me, therefore, having clarified my ultimate motivation; I am then in a position both to improve and strengthen my here-and-now and proximate motivation - so that current behaviour better fits the intended goal. 

How this can work is that - having made this choice and commitment - I can (and should) explore this imaginatively.

...Imagining that I am in the situation I believe that I want; and following-up its consequences and possibilities; to check that it really is what I want; and most want. 

If the initial choice of wanting survives that test; then my motivation is improved by being brought into-line with what I most want; and strengthened by my having explored and affirmed what this wanting actually implies.

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*Any this-worldly agenda is merely expedient; being temporary and destined to fail, because of unavoidable entropy and death - plus, of course, the presence of evil.