You will feel frustr

I am happy that you are back ~ 과 같은 경우입니다.

일반 접속사 that입니다.

* 아래 참조: (특정 그룹의)형용사 뒤 that 절 가능하다고 문법서에 나와 있을 뿐, 형용사구의 목적절이라고는 하지 않(고 있)습니다!

You will feel frustrated (at/ with 정도의  전치사 생략으로 보셔도 됩니다) that you have been working so hard to pay things off and

* 참고 예문: They were frustrated with the council (출처: Collins COBUILD 영영사전) {참고: 접속사 that 앞엔 전치사 오지 않음(예외: in that, except that 등)}

yet you just added more debt to your list

[참고 자료] 출처: http://www.gunaygunaydin.com/FileUpload/ks21007/File/oxford_university_press_-_a_practical_english_grammar.pdf

제목: that-clauses after certain adjectives/participles
The construction here is subject + be + adjective/past participle + noun clause:
I am delighted that you passed your exam.

This construction can be used with
(a) adjectives expressing emotion: glad, pleased, relieved, sorry (see 26 P)
(b) adjectives/participles expressing anxiety, confidence etc.: afraid, anxious, aware, certain, confident, conscious, convinced (see 27). anxious requires that . . . should.
I'm afraid that I can't come till next week.
Are you certain that this is the right road?