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).
It is very good that you are frank about how you feel. You have identified the problem, and have brought it to the open. You are not hiding it from self or others. Suppressed emotions like pent-up anger is not a good thing as there can be times they can collide violently with disastrous consequences. So you have to learn how to deal with your emotions in a skilful way in more perceptive, intelligent way, and see relationships through the Dhamma perspective. Dhamma can be of great help in learning to use the mind wisely.
The most helpful initially is the practice of the four satipatthanas, or establishings of mindfulness. Being ardent, alert, & mindful , helps keep the negative emotions at bay. You begin to develop an inquisitive attitude, to better understand as to why you feel, what you feel at a given moment. Then you get a clearer sense of what’s actually going on, and may even question “Who is this ‘me’ that’s getting worked up.
The gift of forgiveness too helps, if you could instil in self. Then even is someone else upsets you, you will not be a victim for it. This will allow you to see things in another way. So the practice of ‘loving kindness’ meditation helps. Your egoism that makes you manipulative will be eradicated if ‘karunaa’ is to developed. Karunaa is an important part of this spiritual path. In the Gopala Sutta: The Cowherd Buddha has pointed out that:
”Whatever an enemy might do to an enemy, or a foe to a foe, the ill-directed mind can do to you even worse.”
Then in the Kakacupama Sutta: The Simile of the Saw we were advised as to:
"Monks, even if bandits were to carve you up savagely, limb by limb, with a two-handled saw, he among you who let his heart get angered even at that would not be doing my bidding. Even then you should train yourselves: 'Our minds will be unaffected and we will say no evil words. We will remain sympathetic, with a mind of good will, and with no inner hate. We will keep pervading these people with an awareness imbued with good will and, beginning with them, we will keep pervading the all-encompassing world with an awareness imbued with good will — abundant, expansive, immeasurable, free from hostility, free from ill will.' That's how you should train yourselves.