BL, I am hereby giving you permission to leave. You've tried hard and so has Alex, and you're both to be commended for that. Choosing, in the end, to leave doesn't invalidate or diminish the hard work or the love or the time spent together thus far. All of that is still real. But what I'm hearing, in very clear terms now that you've had time to go over it more then once, is that you're ready to go. I actually mistyped that as ready to "grow" and I probably needn't have corrected myself.
You know what you want and you know what you need to do to get it. You want freedom and space and to be solo while exploring new loves. That is ok. You've held yourself back while you've ben slipping into depression because of your love for Alex, because leaving didn't seem fair, because you knew it would be hard, and because you didn't know if you had the right.
It *will* be hard but it will get easier with time. She'll be ok and you will be ok and you'll know that you finally chose to do the hard thing rather than second-guess yourself and swallow yourself back indefinitely. I'm telling you now, you have the right and it is ok. You can leave any time you want. You are free.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.