Cheating

My ADHD husband and I have been married for 20 years. We have had our ups and downs but the last year has just been a down.  Neither wants to call it quits and I’ve signed us up for the marriage seminar.  However I can’t shake this feeling that my H hasn’t been faithful.  I have asked and he denies it.  Still my gut says he has and as I get older my gut instinct are pretty good.  I need my husband to understand that I need to know what we are recovering from. With this suspicion it really puts a damper on my willingness to work to make things better. He has lied to me before about various things with little remorse. Any suggestions on how to get the truth? I’ve even considered hiring a PI but that seems crazy.