So much of your mental state is about your internal dialog, which is something you can control if you work at it. Consider looking at a book like "What to Say When You Talk to Yourself." http://www.amazon.com/What-Say-When-Talk-Yourself/dp/0671708...
Also, I've found that when I'm down about something, the best therapy is just to immerse myself in something. A startup project, exercise, whatever. This might be a good time to just dive into the startup with even more intensity than before. If nothing else, go with the old-fashioned meme of "the best revenge is living well." Picture yourself cruising in your new Ferrari and passing your ex somewhere and just smiling at her.
Or you could dive into some exercise routine / hobby. If you mountain bike or rock-climb or trail-run or anything fun, just start doing more of it. Find some other people to ride/climb/run/whatever with, so you have people to talk to and keep your mind off of the ex.
And finally there is the old PUA saw of "GFTOW" (Google it if you don't know the acronym.) Sounds kinda vulgar, but sometimes it's what a guy needs.
- Change: change is a sequence of stages you move through according to well-defined processes: (1) pre-contemplation (where you haven't started considering change, or are even aware it's an option), (2) contemplation (where you're considering change but haven't decided), (3) preparation (where you've decided to change and start preparing for the consequences of change), (4) action (where you actually practice change) and (5) maintenance (where you maintain change - going to gym once is change, but doesn't really count :)
That 'instant' you refer to is familiar to me and IMO it is when your brain collects enough awareness of your push/pull factors to move you from preparation (3) to action (4). Many people underestimate how critical preparation is, and for most people steps 1, 2 and 3 are not conscious at all and you will go back and forth over them for years. If you're quitting smoking but smoking gives you a break from your annoying boss, or it's how you socialise, or gives you access to the cute guy/girl you're into, and you go unprepared (unaware) into action and quit smoking you are highly likely to relapse (i.e. go back to stage 1 or 2).
I highly recommend reading Prochaska et al.'s "Changing for Good" (http://www.amazon.com/Changing-Good-Revolutionary-Overcoming...) to understand about the stages of change, how to evaluate where you are w.r.t. a certain change and if you're ready to move forward to the next stage, and the processes that help you move from one stage to the next (taster: 'commitment' is only suitable from the 3rd stage onwards, and can actually hurt your chances of successfully changing if you commit when you're in an earlier stage!).
- Changing the way you think: David Burns' books, particularly the "Feeling Good Handbook" (http://www.amazon.com/Feeling-Good-Handbook-David-Burns/dp/0...), and Helmstetter's "What to say when you talk to yourself" (http://www.amazon.com/What-Say-When-Talk-Yourself/dp/0671708...)
- Awaken the Giant is a great book, and I get the timing thing you talk about. I read it first over ten years ago and it meant nothing to me. Having gained a bit of experience since then I listened to it recently about a month ago and suddenly I could relate to 90% of what was being said, and Robbins helped solidify a bunch of ideas that had previously just been floating around in my head.