I went through this very thing, and yes it is HELL. Things he was doing and saying weren't making any sense. Like saying he'd been at the gym for several hours but his gym clothes were as fresh as if he'd just put them on. But when I would question him, he'd have an answer that sort of made sense, and made me feel confused and stupid. Lots of little things... none of which on their own would have been a big deal, but all together it was becoming a HUGE deal.
I KNEW he was lying to me. But I didn't know for sure about what. And that's what was making me crazy. The marriage was becoming so miserable that I wasn't sure I could continue even trying when I knew he was being dishonest about something. I definitely had strong suspicions that it was another woman, because when you think about it, the list of possible things causing your spouse to lie to you like this is pretty short. An affair, a gambling problem, a drug problem, committing crime... what else is there, really?
I knew what it was, in my gut. But I didn't feel able to end the relationship or even declare an ultimatum without some kind of proof. I remember telling my IC, that if he was having an affair I wished I would catch them in bed or something, because at least then I would KNOW what I was dealing with and what I should do. Because like you said, the not knowing was making me crazy. Being SO sure, and yet not sure at all... it really does mess with your sanity.
In the end I got my proof, and I'm glad it wasn't catching them in bed. It was a very sappy, I love you, you mean the world to me, we're a perfect couple blah blah blah Valentine from her to him. I seriously felt like I was going to throw up. But underneath that was a tiny bit of relief that now the crazy not knowing was over. I wasn't really that surprised when I found it, except for the signature, because it was a name I know, a woman I know.
Anyway, sorry this is so long. I just wanted you to know I understand, and I agree that where you are is indeed Stuck In Hell. I understand the feeling of being unable to take action and being so frustrated. Because you have just enough information to make you miserable, but not quite enough to take any definitive action yet. My advice is to keep sleuthing, until you find what you need in order to confront him and take action.
It feels horrible to be so suspicious of someone you love, someone you've promised your trust and faith too. But please, don't feel guilty. If he's given you good reason to feel the way you do, then you owe it to yourself to find out the truth.
Me: Enjoying life
Hugs to you, hang in there!
Him: Someone else's problem
Life shrinks or expands in proportion to one's courage. ~ Anais Nin