Why is my battery dying so fast?

i have a t-mobile samsung galaxy s3 ive had it for about 7 months i don't know if its my battery thats acting up or the place you plug your charger in but i can take my phone off the charger and it will be draining so fast i have to turn my phone off so it can charge faster but only give me a little life i downloaded many apps to fix my battery and to see if that was the cause of it i use them but no improvement before i could be on my phone with it charging and it would be fine now i use my phone while its charging instead of it charging its draining all the way to zero i don't know what to do this sucks can someone please help me someone that had or has the same situation with the s3! What can i do i don't have the money for a new phone i just need to know something thanks

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