Hi! I not sure this is still actual, but I just saw your question and since I'm in the process of writing the personal statement for MSc Psychology myself I decided to answer you :).
I can imagine it sucks when the only things you can think of are the classical cliches. What really helped me was not staying with the first answer I gave myself. That is when your first answer is "...because I want to help people" then just ask for more: "why do I want to help people, what does it give me?". Then you answer something like "I feel my life is more meaningful when helping people" - "why do you find the meaning in helping people and not in helping something else" and so on and so on.
I mean be hard on yourself, don't be content with the answer until you know that the answer truly explains your own individual motives. It sort of makes sense to dig deep into your motives and thoughts especially...you know...when applying for psychology :D. Hope it helps!