I think it’s handled well, but you have to really
read between the lines.
The tl;dr of my interpretation is that Starswirl disappearing left Celestia in a state of arrested development. She never built a support network, and she never gained a sense of independence. This meant that all she could do was defer to others. There’s a recurring theme of “sacrifice” with Celestia’s character. That, combined with her lack of independence and support to fall back on, is why she couldn’t save Luna. And she suffered for it.
So when Princess Celestia received that letter all the way back in Season One, she knew that all of her past demons were going to resurface, and that she needed to make sure her student had what she lacked. So Twilight was sent to Ponyville and the rest is history.
She was able to succeed where her old mentor had failed. After our dysfunctional princess had kept things together for 1,000 lonely, guilt-ridden years, she gets to leave the kingdom in more qualified hooves, retire, and just be with her sister.