Cell Phone Problem At the End of the School Year

Since I've had to charge my cell phone battery for about 7 hours between yesterday and today, I knew that something wasn't right. When I first got the phone as a surprise Christmas gift, I only had to charge it ten hours the first time- as it was brand new. After that, I could charge it for an hour or two, and it would be completely charged.
My dad, earlier this semester, suggested that I try an experiment. He wanted to see just how long my little phone could hold a full battery charge. On the days I didn't have school, my phone was on for five or six hours. The charge would last a week, two weeks, three weeks, a month. Since then, that's no longer the case with the battery. It's old, and not keeping a full charge let's me know that Dad's experiment no longer works.
I e-mailed my mom to let her know that my cell phone no longer holds a charged battery like it used to. She told me to try to deal with a half-functioning phone as best I could until the end of the school year, and she'll replace the phone. I'm glad she didn't try to suggest only replacing the battery, as I can't charge one battery with a replacement battery in the phone. My dad's phone isn't like mine. Our phones are different, and that makes it quite challenging for me to deal with a battery issue.