Hi nak, it's completely understandable that it's going to be painful to think of not seeing her/her not being close to you, after everything that's gone on between you.
But maybe just try to focus a bit more on the good times you've shared and allow yourself to smile at what you've had. Nothing can take those memories away from you, they are something to be cherished/to hold close to you. No forgetting them, right??!!
And as for saying thankyou, yes, that can sometimes seem a bit "empty" (?) but it can depend on the feeling/emotions you put into saying it that can make it mean so much more. And I know you're going to be saying it with meaning!!
Then from what I remember you have let her know how important she is to you (mostly!! ) so she is going to know how special she has been in your life. Now that in itself is going to let her know how much she is appreciated. You can always repeat some of that if you feel able......but if not then she likely knows anyway.
And maybe you can send her a present, something personal/meaningful to her??
In terms of her leaving though, I know you'd want the best for her, so perhaps think of it as something she really needs/deserves and try to be happy for her, to keep hoping things are going well for her??
She will have given you plenty that you can draw on when times are hard too e.g. faith in yourself, strength, resilience............(??). So while it might not be at all easy for you, perhaps try to use some of those things? And just by using some of those things it can keep her closer to you as well.
And you know you've got us for support as well, so don't hesitate in telling us when things are hard for you!! Here to support you!!
Alison