I(34f)have my heart broken today after my fiancé (36m)gave a very sentimental gift to a co-worker.

I(34f) have been noticing that my fiancé (36m) is becoming different around me,not bad diferente but more caring when his around me and I know him well enough to see right through him( we've been together for 15 years). Sorry for the spelling English is not my first language, a year ago my fiancé started this new job and we were so happy because is doing what love, from the get go he starts being "friends"with the interns and one in specific grab my attention,she is very shy and likes him a lot because he kinda protect her from some terrible guys on his work place then she made a origami and gave it to him and he puts it on his desk,in a photo he showed me from his desk I saw it and he told me that story, i didn't get jealous I got concerned because she is very young and he just started the job. We all were young and infatuated with a older hero kinda man and that scared me, because it could cost him his job if she got the wrong Idea,and I told him so,he said he sees her as a little sister and she knows that. Well cut to now a year in,he talks to her on the phone,he gets her Ubers and worries if she gets home ok,he watches what she tells him to he listens to the songs she likes and all of this I only know because he lets out a thing here and there but today he broke my heart even more than he has been all this time. I been noticing his been very veeeery nice to me more than normal and today he got one of his favourite books and put it on his backpack,I asked him why than he said "there are some people in the company that loves to pass books around for everybody to read,so i thought that some people could like to read this" I immediately knew it was for her,I just knew, at this point I no longer am scared for him I'm very angry with him and his behaviour,so I told him that I thought he wasn't telling me the truth but he assure me that he was. In his lunch break he texted me saying this "baby let me tell you, today is so and SO's birthday and I gave her the book,I didn't know it so it got me by surprise and I didn't go to her birthday lunch so a gave her the book" My heart broke in half,then I said that I knew that he was taking the book to give it to her and he admitted it... He said that he knew that it would make me sad and he dint want me sad. but here is some context he NEVER gave ME a birthday present,never, and I blame myself for letting this behaviour slide, but shit that hurt. I have a lot of other examples of his relationship with this girl that made me suspicious of him,is a year worth of storys and the post would be vary long. At this moment he is at work and I don't know what to do, what should I do? I need some clarity right now.