I met my boyfriend five months ago on a dating website. He has lots of friends that are women, some a fair bit older and over the last few months I have got to know them really well and would now class them as my friends too as we speak outside of my relationship. He has this one friend though that is about five years younger than him. She has a boyfriend but is constantly texting him/snap chatting etc At times he has described her to me as being 'needy'. Time of the night etc doesn't matter, she'll just contact him. So... I've been a bit suspicious and about two months ago read his messages which proved to me they've had an ongoing sexual relationship for around three years - ending roughly before I met him. I know I shouldn't have read anything and that day I completely went off the wall - I'm on anti-depressants already. I didn't tell him what I knew just that I wasn't happy with them having any kind of relationship. He maintained throughout the conversation that they hadn't had this kind of relationship and I couldn't tell him that I knew. He says he won't choose which I know I shouldn't expect him to but I feel he should if I'm honest. About a month before this he had gone out with her and not find home till nearly 2am... Anyway fast forward to last night... I've been told by a few of his other friends to 'watch my back' with her, despite them all being a bit two faced. She came out last night on our group night out. She'd barely make an effort with me, just smiled and laughed with everyone else the whole night. The atmosphere was very sharp. She was quite flirty with my other half who in turn was an awfully lot more touchy feeling with me than he usually is in public. She did a couple of things like pretend stuff has spilt on her too and wiped her boob right in front of him, followed him nearly every time he went to the toilet and tried to be as close as possible to him. He on the other hand was a bit off towards her, I thought, although in a friendly way. One of his friends whose birthday it was commented that she didn't even know why she had come out and could see everything that I could. I'm totally in love with my boyfriend, he makes me feel amazing, he looks after me and had generally made such a positive impact on my life. We've booked two hols together, one with our kids, and he moved in with me about a month ago. I can totally see us having a future together, something I could never see happening after I divorced my EA exh nearly five years ago. We are really involved in each other's lives from family and friends to study and work and it's all good - but then there's this. Her. I don't know what to do - it's killing me every day but I want this to work so badly but she just feels like a shadow over it. I can't tell him I've read his messages or that would most definitely be 'it'. How do I deal with this?