Is success in this world due to effort and striving, or is it solely by the grace of Allah, and how can one reconcile these two matters without feeling ashamed of attributing effort to oneself or diminishing the value of work when attributing it to Allah, and is this line of thinking a sin?

These evil thoughts, where one attributes Allah's blessings to oneself, are from the whisperings of Satan and require a faith-based cure. For every good that a servant attains is by the grace of Allah, and even if the servant exerts effort through means, the means alone would not be effective were it not for Allah's will. And thanking Allah for blessings does not contradict utilizing means; what is forbidden is relying solely on the means and forgetting reliance upon Allah, or forgetting to thank Him and attributing the blessing to the means after its attainment. Attributing blessings to oneself while forgetting the Bestower is a type of ingratitude that leads to the removal of the blessing and corruption of the heart.