mel I rally don't think you should blame yourself in any way here. if your dh hadn't done anything to spoil the trust in the first place then you wouldn't have any reason to feel like you do
could you just ring him and ask him to come back? tell him you need a hug and you want to sort this out?
would you perhaps consider counselling for yourself? either relationship counselling for both of you or just one-to-one for you? it could help you work out what you want and it could also help you deal with all of these insecurities so that you can cope better with whatever the future throws at you.
i need to say this too - even if she does text after 11pm at night, why would she text "what you up to? x" why is she texting a bloke who's working for her in that way? i just don't like the sound of it. that's not a follow up to work. that's friendly, and looking for a 'chat' if nothing else - but why? like you say maybe because she has her own relationship problems so was looking for attention from your bloke... but i really don't think you overreacted. especially in the circumstances. honestly i don't.
but what i want to say - is quite often, people who are 'up to no good' so to speak, are so convincing that they often have their partner doubting themselves! i've seen this first hand more than once. the person having the affair (or whatever) makes their partner feel like the most worthless, suspicious person in the world. makes them doubt and hate themselves. and generally leaves them feeling low, depressed and confused. it's only (much) later, that the truth is revealed and that person finally breathes a sigh of relief and realises that they were not going mad after all!!
now i'm not for one minute saying he's having an affair. no way. and fwiw i don't think he is. but at the same time, his own actions have spolit your trust in him, and now he's making you feel horrendous for being insecure. that's so unfair. i will say again, this is not your fault and you're entitled to feel how you do. the least he can do is help you work through it together so you can both be happy in the end.
{{{hugs}}}