What should I do if I fell in deep love with my thaiboxing trainer?

He’s 10years older than me, im 18 and he is 28. The problem here is that he has a girlfriend a relationship for 8-9 years. And its been 3 years simce they decided to live together. I feel bad for his girlfriend to continue flirting with him but he never told me he has a girlfriend. And he flirt back. What should i do? Should i get over him or continue on this road?
What should I do if I fell in deep love with my thaiboxing trainer?
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