ไม่อยากทักทายแค่ในใจ แต่จะฝืนได้สักเท่าไหร่

I'm still confused with the meaning of ฝืน. What's the meaning of it in this sentence and can you give me some examples with it???

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It means he (the person from the song) wants to talk to the girl, but he has to keep it with himself no matter how bad he wants to do that.

So, ฝืน is when you 'have to' do something that you really don't want to.

For examples,
- ฉันฝืนใจทำงานจนเสร็จ แม้ว่าฉันจะง่วงและไม่อยากทำ แต่พรุ่งนี้ฉันต้องส่งแล้ว
(I have forced myself to finish the work even though I'm tired and I didn't want to do it, but it's due tomorrow.)
- เขาฝืนใจบอกเลิกกับแฟน แม้ว่าเขาจะยังรักและไม่อยากจบปัญหาด้วยวิธีนี้เลย
(He forced himself to break up with his ex even though he still loves her and didn't want it to be end this way.)

As you can see from the examples, we usually say 'ฝืนใจ' to make it sound more natural.

Hope this helps and doesn't get you more confused xD