Hi,
Welcome to the site!
Sadly, I have to give you some bad news. I would say that there is a 95% chance that your battery is very close to its end of life. Three of my last four phones had a similar problem after a year and a half. Fortunately, I could simply buy a new battery and plug it in, since I always buy phones with removable batteries. You, however, are not so lucky.
It could be that recalibrating your battery will clean up the percentage reading (you can Google how), but this may also hasten the death of your battery. In your position, I would try to find a local repair shop that will replace the battery for you. Make sure you ask about guarantee (since the touchscreen can be damaged when the phone is opened) and also make sure that the work will be done locally so that you are not weeks without your phone. I would also make a backup of your data before proceeding!
Sorry for the bad news and good luck.
Regards,
Eric.
