Since the past week or so, I've noticed my battery life has significantly decreased.

I used to get around 1 1/2 - 2 days battery life, where I'd use the phone normally, surf the web on 3G and things like that.

Now I'm lucky to get 4 hours battery life on 2G, my brightness all the way down, and I'd only turn the screen on the check the time or send a quick text, so the phone would be on standby with the screen off for a majority of this 4 hours.

I bought a new battery thinking my old one was dodgy, but the exact same thing happens.

I've also noticed that my phone is quite sluggish and apps are frequently crashing.

What could be the problem, and how could I fix it?