I know it may be difficult, but I'd try sharing with her exactly what you said here: "It’s like my processing only kicks in after the session so I feel abandoned with whatever unpleasant emotions are there. I would feel stupid contacting her for something that seems so unnecessary but it is really affecting me and making me resistant." Or what was in your initial post.

See what she suggests. She may be completely fine with a check-in when you're feeling that way. I know I've emailed both my former marriage counselor and my current T after a difficult to make sure they're still there. Even something as simple as "You're still there, right?'

Maybe you could figure something out where you could ask something like that and discuss what would feel good as a response. Like would the words "I'm still here" feel helpful? A different wording?

Another option (either instead of or in addition) is a transitional object--small item from her office or maybe a note she wrote saying "I'm here," similar to the Post-It note that Lost mentioned. To help remind you of her presence. But I know, for me, that alone isn't necessarily enough if I'm triggered by abandonment feelings. So I'd discuss it with her, even though I imagine it will be really difficult. I'm sure she wouldn't want you to feel abandoned.