Communication is key to any relationship. It may be hard at times to speak your mind or tell the person you love how you truly feel but this is the key to a healthy relationship.
If you feel like you can't talk face to face, write them a letter. But give them the letter face to face, it's important because sometimes a person reading someone else's words can have a different interpretation than what you are really trying to say.
If you are in a heated argument take a break. Walk away from each other and clear your heads, you don't want to say something you will later regret. Once your head is clear sit down and talk calmly. I can't say it enough, but it is truly important to truly talk things over.
I have learned over the years that no matter what kind of relationship you are in that it's important to talk things through. If you don't talk and let things get swept under the rug then it will only come up later. The truth and feelings always have a way of finding themselves and it's only best to let them out in the moments that count. Anger is a heavy feeling and the more you keep things inside the larger the anger will grow. This dooms disaster for any relationship.
Communicate and you will find the key to happiness.