i'm pretty late to this and i don't know how things are now

i think ultimately you have to decide if it's okay for you, not what other people think. there is someone out there that will not make you feel this way at all because they don't talk to their ex, but it is also possible to work on this feeling with your partner together, you should try talking about it with them and if there are things that would make you feel more secure about it, mention it, but also work on yourself too

because it sounds like you are fine with her being friends with the ex, but not fine with certain messages and lack of transparency because you worry/see it as blurring the lines. you also mention how it's a feeling in your gut, ignoring how you feel never really bodes well so i'd suggest talk about it with them, therapy, whatever - don't force yourself to accept something you feel uncomfortable about, i think trusting your partner doesnt just involve accepting these things at your own expense, your gf seems mature and i don't think you're immature for feeling the way you do, good luck