I have been fortunate in my life to have not lost anyone close to me so far. The closest I had was my grandfather. But I didn't know him well and while I wish that I could have had more time with him, his death was not a huge factor in my life. Therefore, I cannot truly say what you are going through and I'm probably the last person able to give advice. But...
It sounds like you lost him at a pretty young age. He was only about 4 when he died, and I assume that you weren't far off that age, right? Losing someone at such a young age can impact us even more than at an older age, when we understand more.
You say that you cover your real emotions so that others- your family and friends- don't know exactly unhappy you are. But this is not the best action to take. Chances are that they realize something is wrong. You can't really hide the way you feel. We show it in our eyes, and our body language. You may smile, but that doesn't mean it reaches your eyes. And lying to them about how you feel will only make you feel worse. You're putting a stopper on your emotions, and acting happy for others, when that's the last thing you feel. Eventually, your supressed emotions will swell to the point that you really can't keep them in.
Talking to your family and friends is one course of action, and it's usually a really soothing one. Remember that, at least for your family, that they went through his death, too, and probably felt some of what you do, even if they weren't as close to him. Talk to them about how you feel, and then talk about your cousin. Remembering someone I miss always makes me feel better. Talk about their little odd quirks, or perhaps how close you were to him. It doesn't do you any good to attempt to forget him, or bottle up how he was a good friend.
Talking to a third party- such a counselor- is a good idea, too. Perhaps what you can't, or don't want to, say to your family, or friends, you can say to them. They're unbiased, and are there only to listen, and to help. It doesn't make you crazy to talk to one. They didn't know your cousin, and they don't know you, so they have no reason to be anything but a neutral person.
You'll never forget him, or how close you two were, and you'll never really get over his death. But you should be able to smile without him being there. If he cared about you, then he wouldn't want you to be sad over him- he'd probably rather that you smiled.