Catching Them In The Act

HELP!! I semi-recently found out my boyfriend has been cheating on me for months with at least one person for sure. I saw various messages & pictures back & forth between the two of them. I want to find a discreet way to see all of the messages etc that he gets on his phone so that the next time he tries to out & out lie to me about talking to this other girl(because she definitely isn't a woman) I can actually show him proof that he created himself!

I've thought about using a parental control/security type app that sends everything he receives on his phone to mine whether it be text messages, Facebook messenger, pictures, phone calls, or anything of the sort. Being able to see his location would be beneficial as well since I've notes that every time he leaves my house he goes to hers, or she comes to his house after I leave his house.

My only concern with this, is that I don't want him to get notified that said app is running in the background or have it notify him that it is also sending his content to my device as well. I'm also worried that he might go into his settings or just be exploring his phone & stumble across the app because then he will absolutely know that I'm the one who installed it onto his phone & that would just create even more problems we don't need at the moment!

Please, if anyone has any insight on how I can do this(without breaking the bank) or any tips, tricks, or otherwise simple advice on the subject, I would be forever in your debt. I cannot afford to have another relationship fail when all I did was give my significant other everything I possibly could whenever I possibly could. I'm so heartbroken already over this, I'm sick to my stomach & I just want to understand why I'm not good enough in his eyes that he feels the need to entertain other females. I just want to know if there is something I did wrong, or something I could've done better or differently to prevent this from happening because thus far, I've had the same if not very similar outcomes in all but one of my relationships. I simply don't understand why it's so easy for everyone to cheat on me time & time again.

Thank you in advance for any & all help/comments you guys can send my way. It is really much appreciated!