I agree with the people who are saying 'let it go.' I have a friendly acquaintance at the moment who has been pissing me off for ages, for a number of reasons, though I have known her 3 years, not 15.
I have been trying desperately to give her a wide berth, and she keep mithering and texting and leaving voicemails and fussing, and getting others to intervene, to 'see if I am OK,' and she is doing my head in. Your friend may feel the same about you.
I am not saying this applies to you, (and your friend,) but I am giving this woman I know a wide berth because she is intense, we don't have a lot in common, she moans all the time, she makes everything about her, she is nosey, and she tries to compete with me all the time.
I am usually OK with seeing her occasionally and having a beer at the pub with my DH and her and her partner every couple of months, but I don't want the friendship she wants. So I have had to almost ghost her, and don't even want to met at the pub.
I can't completely ghost her, because she lives 15 minutes walk from me, but I do avoid her a lot, and the more she mithers, the more I dislike her and try and avoid her.
In addition, in your shoes, I would have given up by now. I have had similar things happen to me in the past; a friend not contacting me after I have sent half a dozen messages over 3 or 4 weeks.. So at that point, I will stop, and will not attempt to make any further contact.
If they contact me at a future date, I may respond, and reignite the friendship or I may not. It depends on the reason they shut me out. But yeah I have been ghosted too, and I have done it to others to be honest..... It's not very nice to be on the receiving end, and it's not very nice to do it to someone, but often, it's easier than telling someone you don't want to be friends any longer.
But yeah I would back off if I were you...