Neither of them understand it, because neither of them went through the normal formative process of young teens. It's a relationship between a narcissist and a co-dependent that was formed the same time that normal young teens form actual relationships.
The reason she could do those relationship-like things with others, is because those relationships were not this twisted thing she has with MC.
Whenever they both contemplate and say how better things would have been if they were together in college, I cringe. It would have been a nightmare for MC. Mia was just a catalyst for disorders Lacey has been carrying before college. She would have found some kind of catalyst no matter what. MC would have been endlessly cucked in college.
Someone has said her "other side" might be some kind of underlying resentment she has for MC and I thought it's very good theory. It checks out. But I don't think she would have been conscious of it, unless her therapist told her.
I mentioned this a few times along with some other ideas of what the "other side" might be, so maybe it was me you are referencing. However, Lacey mentions her other side twice.
The first time, she hasn't seen what's on the barty video yet, but she's trying to remember and think of what she said, she says the following
L "I don't really know how to make all of you understand this."
L "There's this other side to me."
L "Maybe Mia gets it? But maybe not."
L "I'm not happy or proud of it."
L "And it's not... healthy."
L "I've got so much trauma, so much pain."
L "And the only thing that kept it in check was [MCFirst]."
L "When I left him to go to school..."
The second time, also before seeing the video but this time in her narration, she says
EDIT: Extra lines. The "There are pieces of me that I'm not ready to face" is the original reason I came up with the 'resentment' idea in the first place
LN "A part of me is scared though."
LN "There are pieces of me he's not ready to face."
LN "There are pieces of me that I'm not ready to face."
LN "There are two things that bind me to him."
LN "My undying love and the debt I can never repay."
LN "Even if the unthinkable happened and I somehow fell out of love."
LN "The debt still holds me to his side."
LN "I can never leave him."
LN "He already has to share me."
LN "He has to share me with my past, my addiction, my brain damage."
LN "And without knowing it, he has to share me with my other side."
LN "But he's not ready for that."
LN "He just isn't."
LN "A couple times I almost broke down and told him."
LN "But I can't risk him finding out before he's ready."
LN "Or maybe he'll never be ready. Then what?"
LN "Somehow make sure he never finds out?"
LN "I don't think I can hide it forever."
So all of this is in the context that she believed that she "said something bad or stupid" while Barty was recording her, and that Mia may understand it since she has seen Lacey during college and with MC
If it's resentment, then I do think it's something she only realized it during therapy. She never mentions another side before therapy after all. If she was saying things like "He was only ever there for me when I was with him. He never came to get me, never came to save me" or something like that, then of course, it's not something she'd want MC to hear, and she'd be disgusted with herself for even having those feelings
I do think the other side is more related to self hatred/self harm though. I think it lines up better in that the only way she could feel good is through hurting herself. She wanted to feel worthless, and she wanted MC to get over her. In this case, the hurtful thing she thought she may have said could be something like "Everyday, I hoped MC would stop texting me, that he moved on, but I just kept getting those fucking happy birthday calls.". Obviously, her dominant side wanted MC and liked the messages. However, her self hating side would have been upset every time MC gave her any sense or worth