Why is my phone turn itself off

@gomolemombonami the very first thing I would do is to replace the battery. Of course, you also want to do a reset of your software, just in case there are issues with that.

Use something like this video as your guide.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.