I've said it before and I'll say it again, if you have to ask if you broke through or not you didn't. It's really as simple as that, its something that is so overwhelming obvious when it happens that you'll have no doubt. Questioning if you broke through or not is like asking if you had an orgasm during sex: you don't need to ask, you'll just know when it happens.
Now the real thing is, if your like me and breakthrough before you know what a breakthrough is, it's incredibly peculiar and can leave you in a state of shock for an indefinate period of time. I spent weeks trying to understand what happened to me, whether I had died and came back to life, was abducted by aliens, or whatever; I really had no idea. After finding the nexus and hearing about the breakthrough experience I finally at least had a name to call whatever it was that happened, however even having a name does little for me to be able to explain what it is.
I'll tell you this much though, it CAN feel like your dying. Our natural response to feelings like this is to move in the opposite direction of them, to run from them, fear them. However to really get a breakthrough, you sort of have to go with it, let yourself 'die', so to speak. The more you resist it, the more unpleasant it can be and if you do resist it, you probably won't breakthrough. I guess that's the best advice on it I can give you, you need to know and trust what's happening and completely give yourself over to it, even if your mind is screaming at you that you're dying.
Happy travels
