Why is my phone battery dying so fast?

So... I have a Samsung Galaxy S3 Neo, as I said my battery is draining so fast and I don't think its even charging properly. I charge it at night but when I wake up it is something like 75-80%. Also it won't charge with the original charger. Now I'm charging it with a Sony one I think which has a smaller amper thing (0.75mA while the original was 1mA). Now this wouldn't be a problem to me if it was actually that, but if I play a 3 minute youtube video it drains it like 7%, and when it gets really low (like 20, 15%) it drains SUPER fast, like (15 - 13 - 9 - 4 - 1 - dead). Oh yeah also when it's charging and I unplug it, the battery goes down, and if I keep doing that, it does it until its dead. I think that is about it, I tried using another battery, doesn't work, and I even changed the charging port thing, I don't know what it's called, charging regulator something... and it kept doing that. Please please help, any fixes? :/

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