Absolutely I see where you're coming - you asked what we thought you should do
It's definitely a sticky situation and chas v'shalom, she's not a bad person or "bad" for being female (and pretty) , nor do I think -that you - or her are intentional. But platonic relationships don't happen; 60% of married men have affairs and 40% of women. Just a bad idea. Doesn't mean that you can't friendly, polite, helpful, concerned - but I am not a friend. I do not meet you one on one. I do not have long DMCs with you until late at night. Like the relationship you have with the mailman.
The fact seems to be that you're attracted to her, it just seems like schmoozing with her at starbucks is just a classic bad idea; like Guard said - you have to know where you're holding, and I would never advise any Baal Teshuvah to suddenly "flip out" and banish all of his/her opposite sex friends from their lives. It's a gradual process- I've seen girls do it. Phone call don't get returned, schedules are tight, and if everyone is getting frum together then everyone just gets it. And if there's a memory that doesn't - or shouldn't - go away - one calls a shadchan. I had the zechus of being such a shadchan; though they didn't get married
There's a way of "losing touch" - of course you don't make a declaration "thou are evil female, thou art sinful, banished be thee from mine holy sacred soul!" - or even a nice one.
All true, but I'm not being concrete about your current situation. like Guard, if you're not holding there, live to fight another day, and we all respect you. If you can, try to be hook her up with another source without openly rejecting her or hurting her feelings.
But yeah, it's complicated. You did have a relationship and suddenly whah-boom - but this is how those how-the-heck-did-I-end-up-here stories start.