What is the ruling on continuing a charity that one now feels compelled to give, after it was initially voluntary, and does its reward become lost due to one's vexation?

To maintain a good disposition in spending, bring to mind sincerity and seek the reward from Allah, and reflect on the texts of revelation that explain the virtue of charity. The strong believer in matters of the Hereafter is more beloved to Allah. Make yourself attached to Allah's pleasure, and encourage it towards good with Quranic verses and Prophetic traditions that urge spending and explain its great reward, such as the Almighty's saying: "And the example of those who spend their wealth seeking means to please Allah and assuring themselves [of the reward] is like a garden on high ground which receives an abundance of rain - so it yields its fruit in double. And if it does not receive an abundance of rain, then a drizzle is sufficient. And Allah is Seeing of what you do." The reward for routine giving is guaranteed, and even if no specific intention is made, the mere intention of a Muslim for good is sufficient. However, bringing to mind the intention and sincerity is better and yields a greater reward. Compelling oneself to do good does not negate sincerity.