Weird phone activity

Hi! My current phone is Samsung m21. So what weird activity I have spotted? 

I have some messengers installed, sometimes, I can see messages from people are already read when I didn`t read em 
Also, yesterday my phone just turned off without my action - he just did it by himself. 
My battery is going mad - when I just purchased a phone, he could hold the life for 1 day without charging EASILY! Now it can`t go for even a 10 hours. 
Recently found in internet that my phone can be under spy, but I am too newbie in that, what does it mean? When I googled `phone spy apps` I found a couple of articles and websites (by the way, one of them, exactly this - https://theteensafe.com is offering ALL spy related functions - gps tracking, text spying, call recording and maaany others things for like 2$ dollars (what?) so people instead of making phone defence better for a small amount of money doing spy apps, and charging these money for making phones worth? Where is the logic?
Guys, who can told me how to detect and remove these spy apps from my phone (if they are really installed)? Thanks for any help in advance, I got so many important information on my phone so I don`t wanna drop it or make a full restart, cause in case of full restart I should go for some cloud storage, and this app can migrate to it also.