-Anything that anyone says to your character during roleplay is said to your -character-. It is not directed at you. Do not take it personally.
-If you find yourself in a situation where you find you are unable to act comfortably as your character would, remove yourself from that situation.
-Before you create the character, think about how comfortable you are suppressing your own emotions. Choose a personality from your character that is either very similar or very different from your own based on what you will be comfortable with.
-Try listing out things that annoy, anger and frighten your character. Similarly, list what things your character likes or is pleased by. Decide how your character will react to each of the things on this list. It is easier to react as your character would, if you know exactly how your character would react.
-Use a background color, or perhaps even music, that reminds you of your characters temperament while you play them. This can be a pleasant way of reminding you how you should generally act.
-Keep in mind that just because your character doesn't like someone, doesn't mean you have to dislike them. In fact, your character could even like one character that they play and dislike another one. Remember, the same is true of everyone else too. Just because they are mean to their character doesn't mean they don't like you. Outside of the game, you could in fact be the best of friends.
-Some people like to roleplay conflict situations, others don't. If you are one of those who don't, try to avoid playing a character that will attract conflict. Thieves and 'evil' characters will, at some point in time, have people roleplaying as hating them. Fervently. If you feel that this would upset you, the player, do not play a character of these types. If you are one of those who do like these sorts of situations, just keep in mind that others don't and be considerate within the bounds of your character.