Dh has had an ongoing and imo inappropriate 'relationship' with an OW. He says that I don't trust him and that is my problem. I think I don't trust him because he has acted like a twat so to summarise the points that have dented my trust in him:
*when I had ds he told everyone he was taking paternity leave but actually spent the time in his office with his PA (other staff are based elsewhere so it literally was just him and his PA), leaving me at home on my own with ds most of the time
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his PA would send texts at odd times eg when she was out on Sat nights to say she was thinking of him, etc
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after his PA left the company, she still kept in touch with him, sending texts and calling.
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he would never say he'd been in touch with her but since I sometimes use his phone for work too, I'd see her name in recent contacts, then if I commented on that, he would explain why she'd been in touch but always that it was her instigating it 
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it's almost 4 yrs since she worked with him but she still texts on occasion. Her last text was about a dream she'd had about him - very 50 Shades of Grey
He says that he doesn't encourage her because he doesn't text back and indeed for the last 4 months or so his bills are online and I can see he hasn't called her. However, I think he should have put a stop to all this long ago and since he hasn't then I don't trust him. He says he still talks to all his former staff (yeah but they aren't sending texts saying they dreamt about being Ana to your Mr Grey!)
You don't need to tell me to LTB as this is just one example in a long line of shitty behaviour and I have already decided to LTB but I just wanted some independent perspectives on this 'trust' issue.