Letting go a second time

I had a best friend I grew up with from the age of 1. She was my soul sister and we shared everything, we were really close, I definitely loved her like a sister. And then as we got older, she started to find more popular friends and she started to drift away from me, only I wasn't ready to let go, and it left me feeling so empty and lost. I latched on to the only other friend I had at the time, and she became almost as close as the first friend had been. While I lost contact with the first girl, I never managed to get over it completely, it felt like such a betrayal, especially since she abandoned me at a time I really needed her. The second friend knows all about what happened with the first, and knows how deeply it hurt me and how much I rely on her for emotional support, but I hoped she would feel like she could come to me for help if she ever needed it too. A few years passed and I went to a different school to the second friend, but we remained close, and I promised never to let it happen again. But today I found out she was hiding something big from me, and it hurt that she didn't feel close enough to tell me about it, even though she had confided in all her other friends before me. Now I'm so scared that we're going to drift apart the same way I did with the first girl, and this time I don't have another friend to help me get back up again. I'm not ready to let go of this friendship any more than I was last time, and I don't know what I'm going to do when it inevitably ends. I know this won't sound like it's very important compared to the struggles others are going through but it feels to me like I'm drowning all over again. I didn't know where else to go because the old tree in the back garden isn't quite enough to vent to as I get older, so I came here to see if anyone else has gone through the same thing.