What is wrong with my voice?

I used to have a great singing voice, but as I got older, my voice got deeper & started changing. It seems that every week I have a different singing voice. What should I do when I finally have a singing voice I like? How can I keep it that way? Like, right now, my voice is a good baritone, but I can't sing high notes! If I try, my voice cracks & my voice goes crazy. I want to perform a song for a talent show, but it requires me to sing a little higher. How can I make my good baritone voice a good/great tenor voice? Help me, please!