Why does it keep turning off?

im 14 year old girl, but i have done hardly any damage to my phone (apart from a couple of scratches on the back)

i got my iphone nearly two years ago, and i havent had any problems with it. then, all of a sudden mid use it turned off. i had to plug it in to turn it back on, and then the time and date had been reset to 00:00 Jan 1 2000. It only used to turn off wen i went on the internet or apps, so i only used my phone for texting and calling. But ever since, it has been getting worse - turning off more often. Now, it turns off whenever I do anything! I cant text, call people, go on apps, surf the internet, listen to music.. anything! sometimes it doesnt even turn back on when i plug it in, and i have to wait a couple of days for it to turn back on! I have tried a lot of things.. but nothing seems to work! i am currently using my old phone because i cant take my phone out without it turning off.

also, when it turns off, if i tilt the phone back and look at the screen, i can see an image of what i was doing before it turned off, and then it starts to fade away until i can no longer see it.

i payed a lot of money for this phone, and it is supposed to be a good quality phone. so why is it doing this? please help me!