My car battery is either dying or dead every time I try to start it? Any

I have a 2005 Honda Civic Coupe, I love the car to death, and it is my first car. It is also a used car, I got it from a private owner earlier in October, and the car was working just fine just needed to be cleaned up a bit. However, now entering November every time I try to start the car the battery is either dying or dead. First my dad and I thought I might have left something on, but nothing was left on, and I'm very positive because I always check to see if my inside lights are on, if the headlights are on and such. So then we thought it was because I was not driving it as much and the battery got cold, but that doesn't make sense to me. True, I do not drive the car very often, I probably drive it once a week or a twice a week, but before we even purchased the car the owner and our mechanic said that the battery was good. I'm not sure how old the battery is, but I thought the battery should have been okay. My car doesn't have all those electronic features that newer cars have that would just drain the battery. My dad tried to charge the car with a car charger, but then it still didn't start. However, the dashboard lights, headlights, and radio lights were working though (but it was probably temporary). So I'm not sure what to do, my perfect car is now just sitting outside in the drive way and I have no idea how to bring it life again. Do you guys have any advice or any similar experiences?
Added (1). Another quick note about the battery, I'm not sure how old the battery is, but I would assume that the owner would have probably replaced the battery a few times in her life. I do not think it is a 12-year-old battery, batteries only last for a few years. But even the mechanics said that the battery that is currently in the car was good, so I do not know.

Read more: My car battery is either dying or dead every time I try to start it? Any