I don't think it's quite so obvious that you ought to just leave her already...I have no idea what I would do in your place...it's so sad.
Especially since it sounds like Alex has really been trying lately, reading "Opening Up" and meeting your lover. But I sort of wish she were doing more things to gain her own independence and to find herself, rather than putting all her energy into the relationship with you.
Such a tough situation. When monogamous people want more space and freedom, they break up with their partner and date other people. But because we're poly we don't have to do to that...so it's much harder to figure out when / why we SHOULD leave.
Is Alex not giving you enough independence, or do you feel claustrophobic even though Alex gives you a reasonable amount of independence? Is it the dynamics of this relationship that don't work for you, or would you feel smothered in any relationship?