내 생에 저음으로 받은 프로포즈라 할 수 있었다. = 내 인생에서 처음으로 받은 프로포즈라고 할 수 있었다. => (lit.) I could say it was the very first (marriage) proposal I received in my life. => It was practically the very first proposal I received in my life. * 내 생에 (in my life) 처음으로 받은 (I received for the first time). * 프로포즈라고 => '라고'(shortened to 라) is a particle quoting a saying or a known fact, similar to "that ...". * -라고 할 수 있었다 = I could call it (something). -ㄹ 수 있다 = can / be able to (do something). * 프로포즈라고 할 수 있었다 = I could call it (it could be called) the first proposal = It practically was ... 선남선녀야 = 선남선녀(an idiom for "a good/beautiful(선) man(남) and a good woman(녀)) + -야(ending form of 이다, is). 신수가 = 신수(n. outward appearance - slightly old-fashioned expression) + 가(subject particle). It is usually used as 신수가 좋다 or 신수가 훤하다, for "(You) look great".

I think it should be 처음으로. ~라(서) means because of noun. 할 수 있었다 = was able to do.