Being assertive/manipulative
In Buddhism there is no solution for 'my problem', because there is no problem in the first place. The mind is creating problems out of ignorance, and causing useless suffering. The title says 'Being assertive/manipulative', but as you wrote the people you ask say you're straightforward. Where is the problem? I would suggest to drop this thinking and pondering upon 'I', 'me', 'mine' and focus on the real practice. By using mindfulness ('thinking', 'emotion' ...) and in daily life applying the practice of sila (morality).

Therefore you should know the 10 skillful/unskillful actions and practice them (the skillful ones) actively in daily live (i.e. try to speak in a honest way).