I read your whole post, but I'll admit I didn't read all the replies, so excuse me if I'm being redundant.

It seems, based on your post, that you are/were much more invested in the friendship than she is/was. The fact she was so flippant about your clothes, that you obviously cared about (and we all have material things that, even if it seems silly, are sentimental, important, etc. to us) shows how little she cared about your feelings. It does seem obvious that she was keeping/using them, even knowing you were continually asking about and she was continually promising to send them back. She cared more about herself than you and your feelings. And the fact is: she may have asked about you, but she's probably not losing sleep over the whole situation like you are. Again, not because you're not worth it (it seems like you were a really great friend to her), but because she just wasn't as invested as you were.

Just because you know or are close with someone for a long time doesn't mean you both feel the same way, are on the same page, and prioritize one another the same way. This is true in friendships AND romantic relationships. It's obvious you loved your friend and considered her basically family, and she may have loved you, but just like a failed romance, it just didn't work out because you weren't on the same page.

The last bit of advice I can give you is: the hardest part of situations like this is that you're probably never going to know "why", or "what happened". There are some people and some circumstances that we just never get the answers we feel we need for closure. Thinking about it, going over it over and over, reliving events in our minds, and talking about it to other people aren't going to give us the answers and resolution we seek. It's a difficult thing, but you need to just answer your questions with "I'm just not going to know, unfortunately". And you need to learn to be ok with it.

That said, I had a similar situation with a "life long" friend. We knew each other since 6th grade and were like family, etc. She's been having some mental health issues in the last few years, and we've both had some really difficult personal issues and situations over the years. But, like you described, whenever I was really having a hard time, she would literally disappear. She wouldn't return calls, texts, etc. She would flake out on plans and so on. I just really wanted and needed my friend! But she has always been that way. But, when she was going through a difficult time, she was always telling me how much she needed and depended on me, and how she didn't know what she would do without me to talk to, lean on, listen to her and give advice, etc. And, last summer, we finally had a HUGE falling out, over basically nothing. She flipped out for no reason, at all, and I just had had enough. I can't take her issues anymore. In our case, she was the "drama queen", and I had overlooked SO much of her behavior and attitude and so on because she IS having mental health issues (in counseling, on meds, etc.), but in my case, it just got to where she was contributing NOTHING to the friendship, other than being a burden, creating massive drama, and ultimately, frustrating me, hurting me, screwing me over, and pushing me to the point that I had to force her out of my life.

It was incredibly painful and yes, very frustrating. And I asked myself a lot of the same questions you have asked about your friend and friendship, but I had to accept that I just won't ever know some things. And, she even emailed me on my birthday this year, out of the blue, like nothing had ever happened. I wanted to reply and curse her out, but I refrained. But it really hurt me and made me very angry that she would have even done that. So I get where you're coming from, in a lot of ways.