This is admittedly a bit of a harsh read of Tara, though I think that the fact that Willow actually does have a large amoral streak at his time (because 1) she's had to learn that the rules she grew up with are wrong, and 2) she's never having a full-on reckoning before s6 with the consequences of her actions -- the biggest ones were Lovers Walk, Doppelgangland and Something Blue, but they were smaller) mitigates the fact that I do think Tara is not entirely fair w.r.t. Willow's power. Some of the reasons that I think the narrative justifies that Tara actually does "hold Willow back" in terms of being able to really come into her own in terms of power, IN ADDITION to being a positive moral force in her life (and of course in addition to them both loving each other) is that in season seven, as people not me have pointed out, in season seven The Killer in Me has Willow accepting the loss of Tara running parallel to Spike having his chip finally removed; and (this is all me) in CWDP the First uses the idea of Tara to argue for a complete-giving-up-of-magic for Willow, when Giles and the Coven argue for a more balanced approach, and Willow buys it because Tara metaphorically *is* somewhat of a voice for dialing her power back. These two incidents emphasize that in order to come into her power and save the world, Willow actually has to move beyond constraints related to Tara, though some of that is also more about Willow moving beyond the guilt over how she abused her power over Tara.

Willow's possessiveness of Buffy runs deep, and it stems in great part from fear of rejection -- which, you know, Buffy runs out of town for three months right after Willow goes into a coma and then is in a wheelchair for a while, so, she has reason to fear that Buffy's going to run out on her (she seems to have internalized that it wasn't about her by The Freshman, at least, with the "but there were circumstances!"). She is afraid that Buffy knows Oz better than her, but Oz is also a problem because he makes himself so completely unknowable. She's definitely terrified of Tara knowing Buffy better than her in Tough Love in a possessive way; and she also views Tara as siding with Buffy over Dawn and implicitly over Willow. Which is like the nightmare of Tara and Oz flirting in the classroom; she's always certain that ranks are going to close and she'll be on the outside. While Tara is right to emphasize how understandable Buffy's position is, I also get why that causes the panicked reaction in Willow, because I think Willow viewed Tara as her safe space in which she was able to criticize Buffy for treating Dawn if she felt the need (Tara somewhat criticized Buffy to Willow for not letting Dawn in to do Scooby stuff back in Real Me, so there is relationship-precedent) and without realizing it Tara took that away, leaving no possible room for Willow to disagree with Buffy. Which is, er, a big part of Willow's issues (i.e. not just that she's forever in Buffy's shadow, but that she can't ever be in disagreement with her). (Well, or anyone who isn't a Designated Rude Person like Cordy/Faith/Anya.)