Ah ok - so I completely expect to get absolutely roasted here - but I felt like I should reply, because I've been in a very very (scarily) similar situation, where I'm the 'other woman'.
I was friends with a guy for the last few years. We were FWB, but I always hoped it would be more. He knew this and he knew how I felt - but always told me that he wasn't in a place for a relationship. We texted most of the day, every day. I helped him out with pretty much every aspect of his life and up until a month ago, we were still sleeping together. I was basically his girlfriend, without the title. Then I found out that he's had a girlfriend since before Christmas.
Obviously I didn't know - or definitely wouldn't have been sleeping with him. He even went as far as to have me make a birthday cake for his 'mum' - which turns out was for her all along.
I can see it now for what it is. He enjoys manipulating people and getting what he wants. For me, realising that he was probably telling her that I'm 'clingy and smitten' made me feel like a complete and utter fool. All I'm saying here, is that you don't know what he's telling her on the other side either. He could be telling her what she wants to hear, and she might believe him (even though I'm sure she knows deep down that he's not being completely honest)
I can't tell you the negative impact that it's had on my mental health - and god only knows what impact it will have for me on relationships going forward. It will be incredibly hard to trust.
I'm not saying they are sleeping together, and I'm not defending anyone. I'm just giving my side and my experience.