I've bought a new battery and I'm using the network switch applet to set to 2G whenever I don't need Internet access.
Now I have 2-3 days standby with no internet usage and just a few phone calls.
The sudden power drops are gone too.

Lucky the batteries don't cost much and they are user replaceable.

I'm using mobile phone for at least 15 years and I had no other phone or other technical device where the battery was wrecked after 6 month.

Maybe just bad luck.