That i haven't been a relationship with you is fortunate(felicitating)or regretting. 同学你这句中文是通的,但英文的意思就模糊了!我稍作补充,that(我没和你成了恋人)is (这是幸运(庆幸)的还是遗憾的) 因为(错过你)就要看你表达什么意思?
Missing u 20 years ago is the greatest regret in my life.