Struggling with friendship changes after having my daughter

Possibly a bit of a long one sorry!! I have a few lovely mom friends I've met since having my daughter, and I'm used to my best friend living 300 miles away etc but I'm struggling lately with my closest friend who is not so close to me anymore.... I understand she is at a different life stage to me but that's never really been an issue throughout our years of friendship... until the start of this year when she started dating a guy (was super happy for her to begin with) Long story short turns out this guy is (somewhat happily?) married but she believes every single lie he says to her, even when I have proved to her face that he is still married etc so we had a bit of a disagreement regarding this, but I said as long as she is happy and doesn't get hurt and as long as I don't hear about it as I dont want to get involved. Since this I pulled her up in July to tell her I miss her and it seems that she doesn't bother with me anymore and puts all her effort into the guy and her other best friend. I told her I was upset that she is now going abroad with her other best friend at the same time that me and her were going to go away together, which really upset me as I found out over social media. It's now the end of the year and I barely hear off her or see her at all and even if our friendship doesn't continue I just want her to know it's because she has distanced herself from me. I'm guessing this is quite a common thing but I expected this would have happened soonish after having my daughter, not 18 months down the line? Does anyone have any advice for me or had anything similar happen to them? I'm not sure how to handle the situation anymore and I can't keep making the effort when it really seems like she doesn't care anymore 😭