Is it normal to bite the inside your mouth?

Habits are normal, but please try to stop that.

It happens to me but i don't do it on purpose

It could be some form of "dermatillomania". My brother used to do a similar thing in which he would bite the skin off his knuckles. I would say it's fine until it gets to the point where you are physically hurting yourself. Then you should probably try and do something about it (unless it doesn't bother you of course).

I do it a lot, but I try not to.

I don't do this, but I have a friend who does it ALL THE TIME. And sometimes it gets really ugly. But it guess it's kinda normal.

I do it as a nervous habit or out of boredom.

You should probably see a professional because I did this the worst when I was dealing with anxiety issues and you may be the same way.

This isn't normal perse, but a lot of people do it. I used to. It's a way some people react to feeling anxious, as you figured out. Sometimes you'll notice when you're doing it, but other times you'll do it subconsciously without realizing it and in this way, it eventually becomes a compulsion similar to OCD. There's really no serious health risks to it aside from the obvious soreness and you may eventually get scar tissue if you do it for years on end to a significant degree, so it's not a good thing to do. This can be hard to stop but it is very doable. If you're interested, this site has some more information: www.skinpick.com/lip-picking
Best of luck,
ACE