Boltonrose it is NOT true that you "didn't measure up." You are enough exactly as you are. Getting yourself to truly understand that not just on an intellectual level but on a deep emotional gut level as well has got to be your number one task right now, whether you do it in therapy or not.
The reason I asked about whether the ex might have had any N (narcissistic) tendencies is because a) his hot-then-cold behaviour raised red flags with me, b) you seem to have the kind of self-esteem issues that can make you vulnerable to Ns, and c) once you start getting involved with these types it becomes a pattern all too quickly. Once you've had a relationship with one N I swear, other Ns have radar for you. That kind of "sweep you off your feet then disappear" behaviour is all too familiar and I know from experience how damaging it is.
Definitely Ns can be "in love with love" types. I think it's because those giddy, "in luurve" feelings aren't simply fun, but very powerful, and somehow they temporarily convince THEMSELVES that this time, they will magically be able to have a normal, lasting relationship. Of course they are not really capable of it, and when their own internal dissatisfactions resurface (because you weren't able to cure their narcissism!) -- they may pull the disappearing act. It hurts awfully and you're stuck struggling to make sense of something that honestly doesn't make sense. The only consolation is that, "Wham bam thank you ma'am" is, believe it or not, the least painful outcome of a relationship with an N! Again . . . bullet dodged!
SGB is right that at the dating stage, anyone is free to end a relationship with anyone for any reason. No argument there. However, the difference is that Ns manipulate you and set you up whether they consciously mean to do it (and some do) or not. It's not really a level playing field. If your ex was an N you should NOT feel badly that you taken in. These people are experts at romance (not relationships). The big rush is what they have practice at. They are incredibly charming more charming than any non-N. They pick up on what you want and offer exactly that, whatever that may be. They are the ones who whisper, "I think I'm falling in love with you" in your ear on the second date, who actually scare you a bit with their intensity, who want to see you every day, deluge you with flowers and romantic cards and want to talk on the phone for hours. If you're not on your guard against N behaviour, you actually think you should be worried about hurting him (!!), because he's so mad about you that he seems vulnerable.
I urge you to do some reading and if you think there were any N-ish things about this guy, you need to be aware of that and get yourself some "N-dar," because it indicates you have a vulnerability that other Ns would be more than happy to exploit.