Is a Spying App Too Far?

A couple of weeks before our wedding I found out my husband has a sexting addiction and likes to send pictures of his d*ck to lots of women. He has given me permission to look at his phone whenever, unfriended his usual sexting buddies, and seemed to be really trying for a while. I've caught him slipping up several times. After a couple months of him doing well for a few weeks and then drunkenly sending a nude, I think he has now started trying to hide it again. Deleting messages and conversations, messaging women when I'm not home, using SnapChat with female friends. I'm pretty sure he's still sexting people even though I can't find any solid evidence and I'm starting to feel frustrated and paranoid. (I hope I'm just being anxious and paranoid). I'm considering downloading a secret app on his phone and paying for one month of social media/sms monitoring. If I'm just being paranoid then at the end of the month I'll tell him about it and apologize and I'll feel a lot more secure in our relationship. But if I find what I think I'll find.....Well, I haven't decided if I want to give up and just leave, or drag our butts to therapy. I really want this to work out, but I don't want to be with someone I can't trust. I guess I'm really just hoping this will prove if I can trust him yet. Am I being unreasonable? I could use any support or loving advice you can give