I painted my room with a textured roller. It looks horrible. I want to make it flat again.The difference in height is no more than 4-5 mm. Can i just put a lot of primer on it and it will flatten it out?

how are you going to prime between the high spots and not on them. This would be the only way that you will fill them up.
Of course prime would make the uneveness look smoother, but not by much.

If your not happy with the texture, which is a common issue because its not as easy as you see on the home shows to make it look right. You can always re-texture it either by going side to side with the roller and use less contrasting colors. or simply paint with a low sheen color paint on the walls. Remember the high sheen colors will bring out the uneveness of the wall.
You cannot sand it, so do not even ask. This paint will remain flexible for many years. Sanding will only cause it to smear and look worse.
If you really do not like the look at all, you can always use a smooth sizing paper. Its similar to wall paper goes on the same. But is smooth. This will cover all defects in the walls and allow you to paint over.