help advice plshhh😩🌸😔

I've been dating a coworker for about 6 months. I knew of his ex ! he told me they were off and on . but currently off...  o I believe him . I see that they were spending a lot time together. he will take her on trips with him and spend holidays with her. but will constantly call me in late hours to cuddle with me. I really want to get over him but it's hard because I work with him I just don't know what to do . I neverrrrrr been in this situation before 😟. like how should I treat him at work? if he was to try to make some work with me , how should I respond???