I massively sympathise. I have a lot of trouble with the topic of people not getting back to me, because I feel like I'm losing them. Its like sand running through my fingers out of my hand. I'm so scared of ending up alone because I spent so much of my life being on my own with no friends (I was homeschooled and didn't go out). This fear is gripping me very badly at the moment.
The weird part is that just a few weeks ago I was feeling completely "myself" and rational, and would say something light-hearted like "oh they must just be busy, they'll text soon " with all of the tranquility in the world. Today for example, I was supposed to hear from a friend about meeting up and it got to 3pm (different timezone here) and still no word from her. I had sent her a little nudge in the morning as well and she had been online but hadn't answered. It felt so uncomfortable to be obsessing over it. I knew what I was thinking was irrational but I couldn't stop feeling rejected. I felt rejected and I figured that she wouldn't want to spend time with someone who's feeling depressed and constantly negative. I went through phases of crying and anger.

My point here is that I feel you, and I know what this feels like. I don't know the answer just yet, as I'm still waiting to go to my doctor this Monday to finally get diagnosed. As soon as I find anything out I'll let you know.
Meditation/mindfulness (yes I know its hard to do!) did eventually help me to feel more stable in my emotions and rational in my thinking. I hope this helps.

Big sympathetic hug from Spain.
Ashley x