"Sometimes i feel that it is ridiculous to think that one can overcome lifetimes of karma, with just my volition / free-will / energy / aspiration. Put
another way, how can i think that my thinking/effort is enough to overcome the will of the cosmos/universe/God/karma/whatever-else-you-call-it."
This is an incorrect view of Karma - it is the punishment/debt view of karma - and it is a false view. It is a common view and I am not finding fault
with your particular position here but karma is something else altogether:
It is much more like the resonance that you are most saturated in - you can move past "vast amounts of karma" relatively quickly because it is not an
accumulation but more like a proclivity and patterning - patterning often willfully compressed sometimes outrageously so.
But it is not debt per se - it is not a sentence in payment for misdeeds - it has no judgement on it.
it is an engine of energy that is driven by the inertia of the vibrations you are habituated to, familiar with and in accord with.
Their is not really any such thing as good karma and bad karma - this is a quaint referral - though I am not interested in arguing the topic - many love
their punishments and love their rewards.
As one moves through the wheel of karma - the spectral compressions heat us and stretch us and compress us and we are in constant refinement. We are the
spirit being distilled in a sense.
In your practice you are not "gaining merit" in the sense of some deliverable purchasing trade currency. You do not buy down your "karmic debt". You move
beyond it - and it is not debt - it does not exist.
Ones entire Karmic Load is zero - ones propensities and the strength/compression/rigidity of them is ones Karma.
Merit is not accumulated and no one can take merit from you and merit does not exist per se - it is of the same nature as Karma.