What does he think of me, really? Please help before someone's heart breaks?

Sounds like he has had time to think about it after saying he only use to like you. However, dont you think if his GF knew what he was saying to you, that she would be very upset? If he does not have enough commitment to this girlfriend, whats to say he wont do the same thing behind your back with another girl, if you ever became his GF. He told you shes the "one" . Is this how he would treat the "one" ,flirting with other girls behind her back? Maybe he just likes the attention from 2 girls. He sounds immature and you should back off and find a polite way to stick to the friend-only type of chats. If he tried to start a flirty conversation with you, have something in your mind ready to say back to him, for example-"sorry i dont feel comfortable chatting like this anymore. Its not fair on your GF, you're not single so i dont want to be speaking to you as if you were. Sorry, hope you understand"