Developing Trust and Keeping That Bond

So my female best friend always compliments me of being nice and a good guy which I truly try to be everyday to anyone. But still she has doubts like she asked me whether I will become toxic in future or not and with that she also said that we have been friends for so less time and anyone in that time will be nice, later in life changes may happen and I may not remain same by that time. But I really really wanna keep her in life, I have said to her multiple times that I am really scared to lose her and would do anything in my power to keep you in my life forever. Hence, what could be my play here and what can I say to assure her that I will not change and make sure I never change myself for her because she really mean alot to me that no one can ever imagine.