I think it all goes back to him thinking of their different statuses and repressing his feelings. Like others said before, they grew up together, so he thinks people will think he's taking the easy way out by simply getting with Tae, even if he loves her for real. Rawee is richer than him but he thinks that he worked their relationship by himself, not through pre-existing connections.
I think when he was in the hospital he used the excuse of her revealing his roots as a pretense for him to be able to say that he doesn't want to be around her. There are lots of different excuses Pit has used in order to stay close to Tae ("I owe it to Khun Nop" "She tried to help me with Rawee, so I can help her too" etc...) even though he showers his sweetness on Rawee. In the end, he's only possessive of Rawee because they have the official status together as a couple. But with Tae, it's in his guts/instincts to be worried about her and react in whatever way that will keep her safe and happy. Plus, he's like Tae. Tae doesn't want to admit to Pit that she loves him because she sees that he's 'Rawee's boyfriend." Well, Pit doesn't want to admit he likes Tae either, not after being with Rawee so long and not after telling her he hates her, she ruined his life, yadayadayada...AND he can't admit it if Tae herself doesn't admit it to him. Tae and Pit are both too full of pride. >.<
Of course, this is mostly speculation with observations, until they reveal (IF they reveal) the reason for them falling out.