She was a link to his past and, he knew without any doubt, had once been his best friend. (3.18)
If Teresa is constantly portrayed as Thomas's best friend in the past, it seems a bit odd that he doesn't trust her. Sure, she betrayed him in the Scorch, but her explanation does make lots of sense—seeing that it was true.
Remembering everything didn't sound good anymore. (5.2)
An interesting twist: Thomas, Minho, and Newt don't want their memories back, because it actually doesn't sound appealing to them. Imagine not wanting to remember the past. Would you be the same person if you lost most of your memories?
"You really want to spend the rest of your lives having no memory of your parents? Your family and friends? You really want to lose the chance to hold on to at least the few good memories you may have had before all this began?" (5.24)
Guilt-tripping the kids into wanting their memories back is just another way Janson represents all that is evil in WICKED. None of this, for Janson, is actually about the kids' well being; it's all about getting what Janson needs.
If everything they'd been insisting was in fact true, he didn't want to face his past even if he could. (5.26)
Again, Thomas's past is filled with instances of him working for WICKED, so to face the past for him sounds worse than not remembering anything about his life. He doesn't want to know either the horrible things that happened to him or the horrible things he might have done.
Alby struggling against an unseen force in the Homestead, Gally being controlled with the knife that hit Chuck, and Teresa straining to speak to Thomas outside the shack in the Scorch. All three among his most disturbing memories. (21.9)
These three memories are the key instances of trauma for Thomas: whenever you try to think of why he's hurting, think of these three events.
It reminded him of the Maze, and a quick flash of the horrible memories of that place went through his mind. (23.8)
It's like a déjà vu moment that makes you want to cry and run away.
He wondered if he'd lived in a city like this before, and if he had, how he could possibly have forgotten the splendor of it all. (24.35)
When Thomas sees Denver, he's amazed and can't believe that he has no memory of any kind of city like Denver. It's pretty depressing thinking about how Thomas can't remember grand and beautiful spectacles that he once saw in his past life.
And a memory bubbled up in Thomas. Something about a fail-safe programmed into his implant to prevent it from being removed. (27.7)
Don't you just hate it when you remember something right when it's too late? This happens to Thomas all the time, and in this case, his mind goes under the control of WICKED right when he remembers that they programmed a fail-safe into his memory-implant.
It's all there for the taking. For the remembering. But then he changes his mind, turns his back on it all. The past is the past. There is only the future now. (28.12)
This mindset is ultimately what propels Thomas forward: he's finally able to look into the future without thinking about the past. If he were to dwell in the past, there would be the chance of him changing his mind about WICKED.
"Even with my memories back I can't think the same way I did before. I can see now that things will never end." (44.18)
Teresa is a rare exception of someone who actually got her memories back. When she does, the world looks even bleaker than it did when she knew diddly-squat.