So I've been thinking about Quasar and Moondragon's relationship as it's been portrayed thus far, and I've really liked what Cristos Gage and the rest of the creative team have done so far. I think what makes the relationship work is that they share things with each other so readily, and that they care enough to ask when one sees something troubling the other. I've long felt those were important parts of a relationship, that you not only care enough to support your significant other when they're feeling low, but that you care enough to notice something is wrong and ask them about it. Without communication, things can fester under the surface, and that's never good.
It's probably a little easier for Moondragon, because a) she's a telepath, and b) Phyla isn't one to hold back on what she's feeling. But by itself, the second one says a lot. It tells us that Phyla feels comfortable enough around Heather that she doesn't think twice about admitting that she feels overwhelmed trying to learn how to use the Quantum bands, or that she doesn't think she can honor her father's legacy. Because she shares with Heather, Moondragon can lend her emotional support. Plus, it gives Heather insight into what goes on in Quasar's mind, so she can tell, even if she wasn't telepathic, when Quasar is trying too hard to prove herself, by charging into a fight she shouldn't be or whatever, and then Heather can rein her in, because she doesn't want Phyla to get hurt.
It's a little more difficult for Phyla, because Heather doesn't seem like a very forthcoming person with her innermost thoughts and fears. But Phyla helps when she can, keeping an eye on Heather, trying to see if those headaches she's getting from picking up everyone's psychic screams of pain are flaring up again. And in response to that concern, Heather's opening up a little more. She believes Phyla cares deeply enough about her that she can mention that she tried to sire a child with Wendell Vaughn before she met Phyla. Or she can talk about how in her earlier years she was very cocky, and that lead to her downfall and eventual demise (which I imagine could explain her closed off attitude. It was arrogance and pride, emotions, that got her into trouble in the first place, so she may be trying to prevent a repeat). Even though she says she was worried she'd be seen as weak for that mistake, the fact she shared that with Phyla demonstrates that she trusts Phyla. Moondragon knows (perhaps on a subconscious level) that Quasar won't think less of her for that, or she cares enough about Phyla to feel that it's important to share this.
After all, Phyla is struggling, not only with the Phalanx and her own doubts, but with the lingering presence of Annihilus in the bands, which is she isn't careful, can take control of her. To Phyla, Heather seems to be always in control, very calm, collected in battle. Heather telling Phyla about the time she feel under an outside influence, and how she eventually managed to overcome it (even if it did require her to die), might give them a deeper bond, which could, in turn, make Heather more effective in keeping control, knowing she has someone who went through something similar.