How do I ask/ get her to be more open sexually with me?

So me 35M, her 37F we have been seeing each other for approx 8 months now.

Background to the title of this post, so at about 3 months of seeing one another where things were in the early days and we still not official she got with another guy briefly for a week where she went to meet this guy in another country where he lived, they had briefly met in the UK for 30mins which was just weird and erratic behaviour!! since then I said the only reason I was still talking to her was that we was not official

Anyway I had a suspicion that she may still be talking to this other guy and went down her phone when she was in the shower, what I sore was literally just when my girlfriend started texting this guy they are talking about having sex within a day, explicit sex acts, sending each other nude pictures, she sent pics of her tits, and a voice note of her moaning then a video of her fingering herself, he send multiple videos and FaceTime calls of himself doing things back,…as I checked more texts she goes on about talking about her sexual fantasy’s and what she really likes sexually to be done to her, however she had stopped messaging him which was a good sign but had left the chat on her phone ( she has not messaged him back since).

I did ask her about this and put to her that I sore these messages on her phone she was rather embarrassed about the whole thing and apologised however nothing much more was spoken about.

My issue is even before this guy turned up briefly in the early days I would try and suggestively text / flirt with her to see what makes her tick and to attempt to lead the convo onto have that sexual chat with her as I’d like to share sexual fantasy’s that she likes and I.

Everything I seem to try before this guy and now I get one word reply’s, my attempts are shut down, I sent a pic of myself before I knew of this guy of me in the shower chest showing I covered my genitals but is was basically a nude pic and I got a one word reply “oh that’s nice” with a heart emoji meanwhile she is sending nudes and getting pics from this guy and entertaining him talking and sharing desires, I have suggestively hinted via text multiple times as she is quite shy talking about this in person and I get nothing much from her.

I noted a few texts she sent to this guy how she can be so open with him, she met him for a total of 30mins in the UK where we live she then got on a plane and spent a week with him in a different country before which sending multiple nudes and videos and sex chat about what she wants and likes yet here I am 8 months on and she can’t be open with me about this stuff, I am really confused and looking for advice how to approach this, do I sit her down for a chat about it?

We still have sex and are intimate but we just don’t talk about it at all which frustrates me we don’t share anything that makes us tick and out desires yet she can with a stranger

TL;DR my girlfriend of 8 months won’t be open with me sexually talking about what she likes etc however is completely open with another guy she met briefly, looking for advice on how to approach this.