Valid concerns or am i just being paranoid/insecure/unfair?

I’ve been with my boyfriend for a little over three years, he’s 19 and I’m 18. We’ve been friends/talking since we were about 13. His family are friends with another family and they have a daughter our age, I’ll call her G. When we were all 14/15, my boyfriend and I had said we liked each other but didn’t get together or anything just kept talking, then in the summer he had a bit of a thing with G, only as far as holding hands and flirty messaging but still some history. At the time I was pretty hurt by this because I’d just said I liked him and then he kept telling me how fun it was holding hands with G and stuff and I felt insecure and rejected. In more recent years they haven’t been as close, but now this summer they’ve been spending a lot of time together as they’re both interested in sailing and folk music and have been going sailing and also went to a festival, with family too. I’ve been staying with my boyfriend since November originally due to Covid and then carried on becasue it’s been nice. Recently he’s been texting G almost constantly throughout the day, but he tries to be secretive about it because he knows she’s a sore spot for me and my insecurities. Him being secretive about it makes me feel like he’s hiding something, and I have looked at their texts sometimes when I’m overwhelmed with thoughts that they’re talking about me or that there’s something between them. I have explained it to him, but not in a very rational way because it makes me quite emotional and feel low and worthless. I know nobody can really know except us, but does it sound like I’m just being paranoid and unfair on him when they’re just friends who share a few hobbies? Is it valid for me to feel concerned about how much they message or does it just make me feel so rubbish because I’m so insecure and feel like she’s more attractive and interesting to him than me. For as long as I can remember I’ve believed I’m not good enough, and I did recently have some CBT in which we touched on that, but it’s definitely not resolved and is not helping in this situation as it makes me feel so worthless and like my life isn’t worth it. I’m going on holiday with my family soon and my boyfriend has already said he wants to do lots of sailing, and I don’t know how I’m going to cope knowing he’s spending lots of time with G and will be too busy to talk to me much, and I’ll probably drive myself insane overthinking what they’re doing and talking about together. I’d appreciate any advice on how to discuss this with my boyfriend in a rational way that doesn’t make it sound like I’m saying I don’f want him to be friends with her, because I know I can’t control who he’s friends with and I do want him to be able to the things he enjoys that I don’t like as much. Also advice on improving self esteem and self worth so maybe I won’t constantly feel inadequate and insecure. Sorry for the rambling and if this makes no sense at all :/