How the hell do I end this friendship?

At the expense of this reading like a reddit post, I'm confused on what to do lol. I've known this girl since we were 15 and 16. Now I'm 21 and she's 20. She's an online friend from a neighboring country, a very sweet girl who even sent me physical stuff a few years ago. We were close, talking daily. The most we'd go without chatting was 3-4 days back in the day, which was quite normal for our dynamic.

But DUDE. Jesus fucking Christ. The problems started like 2-1.5 years ago. She started to disappear for 4-5 weeks straight. She'd reply to my messages, then fuck off for another month, and so on. She came back after like 5 weeks at some point, announcing she's engaged and I was so shocked I accidentally matched her energy and didn't reply for 3 weeks. OK I WAS STARTING TO FEEL STUPID that I'd answer in one day while she'd do it in a month. It went worse and worse from there. Never understood why you'd ignore your friends for a man. l've had relationships before and I literally never had the problem of ignoring my friends

So far, the last time I spoke with her was in July. Beforehand, in may. Sometimes her replies are shorter and more shallow as well. She sent me something 3 weeks ago and I still haven't opened it, because after 1.5 years I started to actually dread responding. We also kind of grew apart naturally, we're having completely different lifestyles, so ig we don't have much in common anymore anyways. I would miss her but I think it's better we outgrew each other organically, instead of having a fight or something

The thing is, how the hell do I do it?! I can just stop responding completely ig...? I'd feel bad tho, after knowing her for so long. Blocking is not an option cause that would be malicious. I'd speak to her, but idk how, I feel like my arguments would sound petty and childish. So yeahhh, I'm writing this cause maybe I need an outsider's pov.