Does performing a permissible act with a personal intention (such as loving one's profession or marrying a specific person) diminish the reward or count as an act for oneself?

The cure for whisperings (waswas) is to ignore them. If you do permissible acts without the intention of drawing closer to Allah, there is neither reward nor sin. However, it is more complete to have a good intention, as a servant is rewarded if he intends by a permissible act that which brings him closer to Allah. Sheikh Ibn 'Uthaymeen clarified that a person should intend several things by his eating and drinking: obeying Allah's command, preserving the body, enjoying Allah's blessings, and strengthening oneself for obedience. Thus, a successful person can transform his habits into acts of worship.