" i dont want to use that function becouse i have multiple screens in my program."
Bad way of thinking... Because using the function is the best way to manage your problem.
If you have multiple screens, you probably a variable indicating in which screen you are.
So, in mouseClicked(), you can check if you are in the right screen before checking if the button is pressed.
Otherwise, if you really want to avoid it, you can have a boolean to check if mousePressed is checked for the first time.
Something like:

1. if (mousePressed)
2. {
3.   if (!mouseDown)
4.   {
5.     mouseDown= true;
6.     if (<the check if mouse coordinates are within the button>)
7.     {
8.       increaseScore(); // Etc.
9.     }
10.   }
11. }
12. else
13. {
14.   mouseDown= false;
15. }

A bit more convoluted than a check in mousePressed()...

thank you for the answer! I havent thought about using the screenstate (which screen is active) variable. thanks allot!