What the ending truly means
Ellie felt like she had to kill Abby because of her PTSD. When she finds her almost dead on the pillars she's starting to wonder what the point is, maybe she's been punished enough, and she has Lev and Ellie is probably seeing a lot of Joel in her with the way she's protecting him. She's about to let them go, but then Joel's dying face flashes before her eyes, and she knows she has to do something, she has to kill Abby or die trying. That's why she starts the fight, because she is haunted by Joel's beaten, bloody dying face and she needs to make sense of it.
But at the last moment, when she's about to kill Abby, it's not Joel's death that flashes before her eyes. It's a peaceful memory of him playing guitar on his porch. A memory about forgiveness. And at that moment she understands that it's not worth it, that this isn't what's causing her PTSD, and that she needs to let go of her anger, the same way she tried to do for Joel. That's why she lets Abby go.
And then when she gets to the farm and plays guitar, it's so not about the fingers. If anything almost every time Ellie plays guitar it triggers a bittersweet Joel flashback, but here it triggers the sweetest flashback of all. That last scene is not about the missing fingers, it's about the flashback. She remembers that the night before he died she decided to try and forgive him for what he did to her, she decided to try and let go of the pain he caused and of her survival guilt. Blinded by her rage after he died so suddenly, she forgot about this, but now realizes it's time to let him go. So she lays the guitar down gently, gives it one last loving look, and leaves without looking back. This is thinly veiled symbolism for her letting go of Joel, of the pain he caused her, of her survivor guilt.
The last shot is Ellie moving forward, most likely going back to Jackson to get Dina back. That last flashback was about forgiveness, and she thinks maybe Dina can forgive her too. Where else would she go with such resolve anyway? She didn't glance back, she just picked up her bag and walked away.
(the part about her going back to Dina is full interpretation on my part, but Neil Druckmann has confirmed in a podcast that for a long time the last scene had Ellie grabbing one of JJ's toys left behind before leaving, implying she was going to try and get her family back. They ultimately removed it in order to make the ending purposefully more ambiguous, but I choose to believe she's going to win back Dina because please don't take this from me)
I think this is the most beautiful ending and character arc ever given to a character in any video game, movie or book that I've ever seen. They didn't do Ellie dirty at all. This is a fitting farewell to Joel and a magnificent conclusion to Ellie and Joel's story. Joel is put to rest, and Ellie can finally live with herself.