[Q] Battery problem... quite bad. topic

I have a Galaxy S3 that I bought 2 years ago in the Samsung Store. Unlocked, of course. I have never rooted it, or done anything like that to the phone. I have been always careful with it.
But I have been having this battery problem for almost 1 year and I'm very tired...
Everytime I try to charge my phone it just odesn't charge, it discharges or charges veeeeeeery sloooowly. No matter if I do it with the phone on or off. When it charges normally I get so happy I could die
Things I have tried:
-getting a new battery
-getting a new USB cable and adapter.
-checking the charging port (sorry I'm not an english speaker so I dunno how to say that)
-erasing all data and leaving my phone as if it was new
-EVERYTHING

What I'm doing to use it:
I bought a universal battery charger. I got 2 batteries. I charge one, put it on my phone while I charge the other. Each battery charges in 8 hours or so, sometimes 9. Yes, I'm suffering. Please, help me. I have no money to get a new phone, plus, I'm underage. I'm a girl.

PLEASE HELP ME!