MacBook battery isn't working ALL OF A SUDDEN!

Hi, I have an MacBook from early 2008. Last week my battery was working 3+ hours without being plugged in. Then I noticed that the battery percentage icon was not changing from 100% and wasn't accurately displaying the charge. When it was close to dying, it warned me (at first) but the icon still showed the wrong information. This morning it worked 3 hours without plugging in although it said 100% for 3 hours then suddenly shut off (without warning). Now it says 100%, green light is on but it's not charged, if I unplug it shuts down immediately.

I know it's an older MacBook and I could understand if the battery life was slowly becoming shorter, but to go from working perfect to dying doesn't make sense. I've taken really good care of the battery/charging since I purchased it (accordingly to apple instructions).

Any help would be great!

I have tried to follow steps to take out battery, power down, etc.

Thank You!