Being assertive/manipulative I am facing this problem currently. I am very sensitive and fearsome. I don't like to hurt others and am scared to argue with anyone. I become very hurt if someone has a bad attitude towards me. This only ends up in my stress levels increasing and losing patience and self hatred.
Colleagues and friends say that I am not manipulative and very straightforward and that is what causes so many problems to me. I feel that I should not care whether others are good with me or are bad with me.
But I tend to become over reactive suddenly and this is creating a chain of problems. I am sure in Buddhism, there would be the way to solve my problem.
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).