Trust issues and cheating

A few months ago.. 7 to be exact i caught my boyfriend sending nudes on a app called whisper and it was not just one or two it was many..

All different girls and him saying he would pay money for things done

He had said to one of them he would get condoms and they could meet up and do stuff.. i do not know for sure if he did go through with it though

There has been more then one time i have caught him but that was the worst.

Once we moved into our new place it stopped he has not done it since but it still hurts...

I still from time to time sneak his cell to make sure there is nothing on it

The worst and hardest part is that he looked me dead in the eyes and promised me he was not doing anything and yet he was..

Well next month he is taking over his bosses job for work and needs to go to philly in order to get certifications and such and he will be gone for a week.. this is causing me so much stress that i am having nightmares of him cheating agian

I do not know what to do about it.. i want to be able to trust him agian but it is not easy

We are planing a wedding and making plans to have our first child and while i do love him.. i am so scared while he is gone something may happen

Yes i know some of you may say leave him but we have so much history and he does treat me well..

I just want to be able to trust him and if not him the next guy if it comes to me leaving but i do not know if i will ever be able to trust anyone..

Is there any advice or anyone going through the same thing?