So to see Hanna actually engage in a therapeutic experience (involving, note, scary warnings and a lot of shadow back-talk; exactly what Hanna would actually experience in this environment) and then come out the other side having gained a fair measure of control -- over the Alison we've all got in there, that speaks for her troubles; itself a major reason they all loved her was that she embodied those energies for them -- is itself fairly therapeutic to watch.
You don't think of Hanna as being the heavy lifter, generally, but that was hard work. It even brings the whole therapy-avoidance thing in, because of how you resist the most right before you break through to something. I don't think this particular move is necessary for the other girls -- although Spencer's going to need something similar just because her rage is so controlled/uncontrollable -- but it's the truth that of all of them, Hanna missed Ali the most and has the most to gain by saying goodbye.
You think that letting go of grief is going to be this sweet goodbye thing and the person floats away into the clouds and a tear rolls down your cheek, but if it were that easy you would just do it. Grief counseling works the same no matter how you felt about the person, obviously, but the toughest ones are the ones that are both, because it's not a quiet sad cloudy tear, it's a burning acid storm.
The hardest ghosts to give up, the ones we hold onto the longest, are the ones that hurt us the most, because they're knit the furthest down into our bones; they hold on strongest, jealously clinging, when it's time to go. When you love somebody, that's about them, but when somebody abuses you, that's about you, so your memories of them are more powerful than they ever were, because it's part of what makes you who you are. Letting go of somebody you both hated and loved means doing major surgery on your own self, burning off the parts that don't work without losing anything you should keep. You can do the easy part after the hard part. This was the hard part.
It's not like anything Alison said wasn't true, although she was exaggerating a tad. But hearing her say it -- the worst things she could possibly say, all at once -- makes everything else that much easier. Because she is dead, and she is gone, and Hanna's still alive. Well done.
Emily: "Okay, wait. Do you or do you not go to this school? For somebody who doesn't go to this school, you sure do come to this school at odd hours."
Samara: "For the record, I have been driving across town for each of our scenes together. That's not weird, is it? Anyway, it was really nice psyching out your mom last night!"
Emily: "What's awesome about your huge lie on top of my huge lie is that you were totally right, and my mom knows it, so maybe it doesn't matter."