Is it normal to act different?

I understand what you are experiencing, though I never cut myself. Sounds like you want to be happy and outgoing, but you have a lot of insecurities. The people around you make you happy and distract you from your problems. You have to start making changes in your life to boost your confidence.

What makes you happy? Do it. Find a way.

Are you a girl? If so, get a couple good friends and suggest you all give each other make-overs(not hair and make-up). This is more effective if you have different tastes. Go to a store/mall/outlet, you don't get to decide what you try on. Your friends do.

Find one really good outfit that makes you feel good about yourself, something to make you feel pretty. Bright colors help. It's harder to wear bright colors if you usually wear dark colors. You may feel and more noticeable. This may make you feel vulnerable. I think you may be feeling trapped under several layers of insecurity.

Start making little changes in yourself. If you look down when you walk, look out and ahead instead. It's going to be harder than it sounds. You may have created a lot of security blankets, things that make you feel safe and comfortable. Like dark clothes, baggy clothes, hands in pockets. You have to get rid of them to feel comfortable in your own skin, because I don't think you are right now.

A lot of people don't know that they can make themselves.
If you don't like yourself, you can change most of what you don't like. You have to learn to love yourself, and be the best you, you can be. It's liberating. Right now your aren't free. You've dug a hole and chained yourself down with self doubt and fear. Things don't change over night. But it becomes easier as you go along. Someday you will look back and say, "Wow. I can't believe how different I've become." It will still be you, just a more at ease, happier you. :)

Hope this helped. :)

Just stop thinking negative it wont do you any good i was like that when i was younger now thinking positive makes me feel happy almost 24/7 and most people have different ways to be when with other people...

Maybe you're just bored with being alone with yourself.

I'm told everyone acts different depending who they are with.

You might not tell the same dick jokes your mum as you would to your friends for example.

Whenever I am down I generally don't show it to my friends. I don't know exactly how severe your negative feelings are but acting happy around friends even if your not is perfectly normal.

Sounds to me like you might have a mood disorder of some sort. It is not uncommon for people with borderliners to act one way when they are in the company of other people who might judge them harshly or even be mean to them whenever they show their sad. They would most likely just say something like this:
Why are you so sad? aren't you enjoying our company? you are such a drag. Cheer up! Or something of that nature. It's not like they would give you any valid advices if you started telling them about your low moods, chances are they'd either make fun of you or start treating you like you are the one who can't put up with sadness and makes a big deal out of it. Or they would just blame your bad moods on you and some intrinsic inability you have to deal with whatever is making you feel that way. Which is a mistery even to you. So you have to put on a "brave face" at least look like everything is okey because otherwise you'd start blaming most of it on everyone else, and spoil the dinner. Then kind of get kicked out. So there is some denial on your part and some lack of hempathy on the other, part. And in the end everyone can keep carefully avoiding talking about real subjects and real troubles making you need to hide your emotions or ignore those who display them.
Now I got to go. The usual stalker.

Have you talked to anyone about this? If you want to commit suicide or hurt yourself than it's not normal. But if you are just thinking about death as an idea I believe it's part of growing up and realizing that as humans we need to place meaning in our lives to continue living as a sane person.

I believe you act happy because you want to be accepted into society. Many people act this way without knowing it, I think you just know the difference between your intrinsic self and your social self. Just make sure you seek help if you are becoming self destructive.

That's kinda how I am. When I'm with my parents, I'm all giddy and laughing, when I'm alone I'm all quiet and just do whatever but once I go to school I'm always happy, I'm rarely sad at school. I just noticed this 'cause one day when I went to school I just wasn't in the mood and wasn't talking and all my friends thought there was something terribly wrong since I was quietxD

You just need to do things that make you happy. Like accomplishing goals and dreams in life can change a different view on yourself, make you much happier that you exist. For me simple things like accomplishing just a drawing can make me feel good. Take some time by yourself and think about the good things, the positive things about yourself that make you, YOU.

Here is somewhat of a different topic but is a long the same lines. The reasons why relationship can go so bad is because people aren't happy with themselve, they think they have to have someone to make them happy. I had actually read some article online that was talking about how divorce rate is so bad: there is a 50% divorce rate in the United States. They said if your not sure if you are ready to be with someone or get married, you need to spend at least a few months living on your own and if your not happy being a lone then you aren't ready.

That's why break up are so tough for certain people. Its because of that low self esteem, you feel empty because you don't love yourself. You have to have someone there around you to make you happy instead of just happier. That isn't good by any means.

I hope this helps. Just keep your head up.

I used to be like that. I still am like that time to time. Life is so much easier, when you force positive thoughts into your head, until they become real!