I have some suggestions for you!!
Firstly, do you warm up enough before you sing? Practise singing high notes often and use lots of different scales arpeggios and breathing exercises.
When you warm up, use sounds like 'mia' and 'ya', and make sure that you smile and drop your jaw - this will make sure both resonators are working.
Then, when you're singing and come to a high note,don't think 'oh no this is high' or anything like that...I know it might seem hard, but try to think lower as you sing higher (and higher when you sing lower). This might seem a little strange, but honestly it really does help. It makes your voice sound fuller and not as squeaky!!!!
Hope that was of any help - good luck with reaching those higher notes!!!
Love Laura
XxX