Got a girl in love with me across the country, how do I kindly shut her down?

Backstory is this girl and I have been hooking up once every month or two since Jan. I used to live in her city and then moved across the country. She came and stayed with me a couple times, and I visited her a few times. Lots of text/sext in between. Exclusivity was never implied and she's cool with that. I like her a lot and I think she fell in love with me a little bit.

I fell off the map and didn't hit her up for almost a month. She said that she couldn't deal with it. I was cool with that, lord knows there are other girlies in my phone that need to be hit up. Now she kinda wants to take it back. How do I tell her no in the most decent way possible?

Girl - 12 hrs 12 mins later: Morning, handsome. I'm bowing out. Still like you too much, and keeping this up is against my rules without a payoff. Let me know when you're in [girl's city]. 

Girl: xoxo 

Me - 2 hrs 44 mins later: "bowing out"? maaan the japanese really got you. :p i called last night to say the same. kick butt girl 

Me - 1 min later: ps. just got your sf card. niiice 

Girl - 6 mins later: Haha, shut up! I'm using formal language cuz it bums me out, dummy. You kick butt, too. 

Girl - 1 min later: Oh, and if you build that time machine, take a vid and send it along, k? Take care. 

Me - 4 mins later: *headbutt* 

Girl - 4 days 11 hrs later: It's even harder than I thought to stop thinking about you. I'll do it if I have to, but can you think of a circumstance under which we could work? 

Girl: This is the last time I'll bother you about this. Promise. It's just hard to walk away from a good thing.

I like her, but there's no real way for it to "work"—I want her to be happy and I'm simply not gonna be present much in her life. If she found a new guy in her city I'd be very happy.

What's the best way to phrase, "I really like you, so you have to stop thinking about me so you can be happy."?