someone is eventually gonna have to go #2

You know how some people can just breakup with one person and immediately they are a-okay and ready for the next new guy or gal to come their way... aS if it was no big deal that they just got their heart trampled on!?! I know people like this. Its like one day they are breaking up with someone and upset and 24 hours later ( or so it seems) they are over it, and often falling for someone else.
I'm not that girl. My friend veronica tells me frequently my downfall has been that i am too loyal to the jerks who have broken my heart.... pining for them and waiting for them to come around, while hanging on to their every word and care for me, ( pretty much pathetically waiting for them to realize they made a mistake... ps.. they always do.. eventually..) I cant even make myself humor the idea of dating or playing the field because I'm so loyal to this person.
yeah , see, I'm desperately trying to figure out how to be that care free girl and i am open to suggestions. Please refrain from sending me the same ol shit that everyone says.. I am fully aware everything happens for a reason. I am fully aware there is nothing i can do about it. Unfortunately the anxiety i feel and heartache has taken over and those things you say wont even phase me. ( probably more so it will piss me off)
I am the first to admit i have been having a hard time dealing with my ex and his new girlfriend moving down here. Moving in with him no less. How the hell do u fall for someone who doesn't live even in your state and so into them that you move them down and into your house. As a couple you haven't even spent more than a week together at one time, - more importantly you haven't even taken a shit in the same house! That's a huge deal i feel like! This is actual stuff i think about. In fact id even go so far to tell them to not even worry about this stuff because I'm worried enough for the both of them.
I just don't get it. I would be freaked about that. I mean for all they know, one of them has an annoying trait that the other wont be able to stand...and then they have moved their whole life to find out what a huge mistake it is! this is very likely it COULD happen... then there is what really gets me... keeping me holding on. The FEAR that it could actually be great. So great that he forgets loving me. So great that he forgets all about the good things i have to offer and the love i have for him and the heartache I'm feeling. this fear could be the death of me. (JK) But this fear is currently consuming my mind, my body and my usually fun loving spirit.
This is where the advice comes in. How do some people just pick up and move on and willingly date other people? How do people even meet people that even catch there interest when there heart/mind/body/anxiety is still on someone else? I wanna move on. I wanna be happy too, but i don't know how to get my mind off the fear that they may work out.- or the fear that one of them may have to go # 2 any day now...