Girlfriend secretly stayed in touch with her recent ex. What to do?

Hi all I (M47) am in a relationship with my girlfriend (F29) for about five months now. We're very much in love (she was the one initiating the relationship btw if that matters) and things are pretty serious between us. However, she sometimes wonders if she enjoyed her 'single' life enough (she's been single for two years since her last serious relationship, but one year feels kind of gone to her due to covid, meaning no festivals, parties, travels etc). I asked her a couple of times if she wouldn't like to be single a bit more and that would be ok, but she insist on having a serious relationship with me. So things are great, but sexually things started out very slow. The first three months it was a lot of hugging and kissing but she wouldn't respond or initiate anything further. Which is ok, I would never rush anything with anyone. But after four months I asked her if there maybe was a reason for this. I asked if she maybe wasn't that attracted to me physically? No that wasn't it. Maybe she had an unpleasant experience in the past? No, also not. Is there maybe still someone else on your mind? No not at all. Eventually our sexlife became a bit better, but she would still sometimes be either totally not in the mood, even after a romantic day/evening, or would completely switch "off" during sex. Stating that all of a sudden she wasn't horny anymore. I found this pretty confusing. She gave fear of commitment/abandonment as a reason and that she usually is very slow opening up sexually in a new relation. But she did used to have one-night-stands or bootycalls with guys when she was single, but that was easier because she would then always be under influence of alcohol or drugs. It almost feels like she has a problem having sex 'sober' or to develop a healthy intimate relationship within a relationship. And although for me physical intimacy is also an important part in a serious relationship, I still try to be patient and understanding. And besides that we're having a great time. Unfortunately I found out by accident in her phone that she is still regularly in touch with her most recent "flirt". A guy she has been having casual sex with on/off in about a 10 month period, ending two months before we got together. After some investigation I found out they met at least twice, they send each other heart emojis under pictures on social media, and he calls her sweetie in chats. He also sent her a message at 02:00am asking if she's still awake. They talk as if they are sharing a lot about their daily lives, meet for a coffee etc. When I confronted her she denied at first and then said she didn't say anything because I would only start asking questions and it wasn't nice for me to talk about someone she had been sleeping with. Then came all the cliche's: I don't want anything with him anymore, he's just a friend, that's just how we communicate, he knows I'm with you now etc. I found out she then immediately send a voice-note to him telling that her boyfriend had been super angry that they had met and that I didn't believe it was pure friendly between them. Ending with a sarcastic "yes really nice.. a relationship!". He replied: "Oh I didn't know you were actually in a relationship with him". This was four months into us being together. Since our first conversation with all her cliche excuses, I feel I won't get any real answers from her anymore. She gets defensive and turns it around saying that I'm making a too big of a deal out of this. She also denies that our 'strange' sex-life has anything to do with this. (Something that came to mind with me as well of course). I decided we should take a two week break to think things over. After two days she sent me a postcard saying that she's incredibly sorry for what happened and that she had no idea that her actions had made me feel so bad. And that she hopes we can work this out because she's very much in love with me and really wants our relationship to continue. What do I do now!??!? I have a feeling further discussions about this issue will be pointless and create more stress. I do like her a lot and wish things could return to normal and we live happily ever after. Or is the combination of her doubt of not having been single long enough (still longs for parties, meeting new people, experimenting etc), our difficult sex-life, and lying about staying close with a recent fuckbuddy enough red flags to just put and end to the relationship? Is this all becasue of our age difference and being in different life-phases and/or having different values? I don't know anymore what I should do. Any advice from an outsider would be greatly appreciated! Thanks in advance

Yep Steven, it helps to share the same values and standards and if you're GF shared them with you, she'd be working with you to keep your relationship together safe, rather than to keep her ex a secret. Sure, she may have missed out on partying time, but if you did it for her, she wouldn't look back over her shoulder. Red flags are waving in your face but it's your choice as to where you go with this and it'll be your ability to listen to your gut which will help you settle it, and regardless of your respective ages.