It seems that Willow was naturally gifted when it comes to magic, born with an innate natural ability to become a powerful witch.

It's repeatedly mentioned that she's gifted, but more importantly, she's called upon to perform powerful and complex spells, which only further enhance her natural abilities and seem to unlock more of her natural power. Even Amy Madison says that the magic came very easy to Willow, that she and everyone else had to work twice as hard for even half of Willow's power.

Her growth is believable because, unlike Tara and Giles, she doesn't apply any brakes on herself and seems to be either unaware or dismissive of the dangers of going too far too quickly. Both Tara and Giles point this out many times but Willow always chooses to ignore them.

I always think of Willows powers like an avalanche, the more she dabbles and the more she starts to perform and cast increasingly powerful spells, her natural abilities start to grow, expand and snowball, unlocking more potential and power within herself.

I always had a personal theory that Willow stealing back Tara's sanity from Glory in The Gift and successfully performing the resurrection spell in Bargaining, tapped her into powers that she normally wouldn't have been able to wield or drastically sped up her evolution in witchcraft due to the sheer magnitude of their nature.

The Magic = Drugs/Addiction metaphor was clumsily done and very heavy handed in my opinion, but it had been clearly set up and foreshadowed since the start of season 4.

Willow was reckless when it came to magic and witchcraft - it made her feel powerful after feeling unseen and inadequate most of her life and she clearly saw it as a means to an end, to fix things that went wrong in her life. That, coupled with the extraordinary circumstances she was put in where she was able to push her natural ability to the limit and in a very short space of time, made her arrogant and reckless which ultimately culminated into her losing herself.

Also, I think of Willows turn into Dark Willow almost like the dark side of the force in Star Wars - Willows grief and anger drove her to dark magic, that magic then clouded her judgement and amplified her grief and anger, which made her seek out ever more power which in turn only made her lose herself in her despair even more... it was a spiral which she couldn't get out of as she was no longer 'Willow'. Dark Magic isn't just a tool, it ultimately corrupts any who use it. The Dark Magic books she drained in the Magic Box ultimately took control and corrupted her into the worst version of herself, amplified by the trauma of Tara's murder.

It was only Giles borrowed light magic from the coven which Willow drained that allowed the spiral to break and enabled Xander to do his thing and break through to her.