I'n not arguing with you when you say Willow is quite selfish but honestly I don't think she knew what she was doing in Tabula Rasa (and which episode it is before Once More with Feeling, All the Way I think but my mind is blank) She didn't realise what she was doing to Tara was wrong in anyway, she genuinely believed that magic could fix everything and that was largely her addiction, not her selfishness I think.
And you think the two are mutually exclusive?
Willow turns to magic to fix things that don’t go her way, whether you want to admit it or not. She’s addicted to magic because it gives her power, it makes her feel in control. She didn’t erase Tara’s memory because she was addicted to magic and couldn’t help herself. She did it because it would get her out of trouble and doing a spell would be easier to fix her girlfriend than admit that she had a problem. The fact that she didn’t realize it was wrong doesn’t make it any less wrong. But the fact that she never fully acknowledged it is worse.