Feeling your emotions is different from talking about them. You can go out in the woods and scream, or chop wood, or run yourself exhausted, or play music. There are any number of ways to feel your emotions. The problem, of course, is repressing them so much that you don’t even feel them or acknowledge them, even to yourself. Holding them in, is not the same as repressing them. You may want to hide that you are feeling anything, but you can still feel it inside.
Expressing your emotions to another person is a strategic decision. You might find it useful to express your feelings to someone so they can understand what you want. You might just want to vent, and go to a friend or a therapist to express yourself.
In the end, it boils down to communications. If you are facing a problem with someone, it may be useful to share your feelings with them. It’s probably more effective if you describe the emotions (“I was really angry”) rather than acting them out (screaming at them). But who knows. That depends on the communication style of the person you are talking to. If expressing your emotions gets you nowhere, or even makes you feel worse, then I don’t see the point in expressing them.
In your family, it sounds like you don’t think anything will change if you express your emotions, and that’s probably true, if all you do is express them. However, if it is a negotiation, and you use your expression of your emotions as a tactic to get what you want, that’s another story altogether.
You feel powerless in your family. You don’t think you matter or they really care, because they don’t change when you talk to them. Perhaps this is true. But perhaps you need to think about this differently. What do you want? Have you ever expressed this clearly? Have you expressed it multiple times?
You don’t have to be powerless. If you learn to express yourself in terms of your goals and what you want and what you are prepared to do to get what you want and what stands in your way (the things that anger you or make you feel unappreciated or uncared for), then maybe you will have much more success. Right now you withhold your emotions in a passive-aggressive attempt to take on all the pain and to feel like no one cares. You might as well be depressed and die. That’ll show them.
You could take a more constructive approach, although that might expose the truth—that it is as you feel. They don’t care. Well, burn that bridge when you get to it. Right now, figure out what you want and express your feelings to help you get what you want.