Feeling like a convenience to my best friend

I'm starting to feel like a friend of convenience for my best friend. we have known each other for ten years, and we can go a while without talking and everything was fine, thats normal for us since im homebound most of the time. She's busy and I completely understand that she has a life and other friends and responsibilities, but I'm always there for her when she needs it. every. time. For 10 years, if she needed me I was there. through depression, a pregnancy scare, everything! the one time I needed her she wasn't there. and I haven't heard from her since (it's been about 3 weeks). I know she is doing fine since she posted photos with her boyfriend just yesterday. she said sorry for ignoring me the day she was supposed to come over (3 weeks ago ish) and help with something important, that she wanted to make it up to me. I told her it's fine and there was no need since I get life can be a lot but I said i was mad for a bit because she left me hanging when I needed her most. I didn't hear back for a week, so I figured she was probably going through something and waited, texting about something else entirely. still nothing. another 2 weeks, I texted again a few days ago. asking if she's mad at me for something. no response. no idea what she could be mad at me for though. then she posts on her socials, happy as can be. I just feel left behind, anxious, depressed, angry, desperate and alone. I'm tired of being a second or third choice. But she's really the only friend I've ever had that stayed by my side even after my chronic illness started. I don't know what to do. am i the problem?