Read what you just wrote OP.
Trying to trust again is hard but monitoring DH phone usage helps... even if building a picture of how much he talks to her.
How on earth can you even begin to 'trust again' if he is talking to her at all ? If you have had to resort to 'monitoring his phone to 'build a picture' of your DH speaking to someone he has been having an inappropriate relationship with, what on earth do you propose doing with that picture? What , in your mind does the monitoring achieve ?
You have two choices.
You end the relationship because he is/has/will continue to be faithless , which in itself demonstrates a disregard for you that is breathtaking.
You stay in the relationship because for whatever reason you justify in your head, you are prepared to accept this behaviour.
What isn't going to happen - is that , having been 'caught out' by your monitoring- he is in some way going to go 'oh User0811 I'm so sorry, now you have proved to me that I've been lying to you, I will stop and be faithful to the end of our days'. I'm sorry this just isn't realistic.
You can continue to monitor and to have His philandering rubbed in your face hourly (I can't imagine that pain, all day every day) or once 'caught' He will just stop using what'sapp and continue as before without your 'surveillance'
Ultimately it's completely pointless , and a huge distraction to the main problem.Obsessing about a monitoring site rather than WHY you are monitoring !
Your efforts need to be redirected to get some serious help in garnering some self respect to enable you to leave.