這用法我之前也是找了好久:p 我找到的並沒有分析單一文法詞性(but what ruke said makes sense)，只是對這用法有介紹(換句話說整個背下來，然後記住整個的意思)
以下出自"Practical English Usage" by M.S., 3rd edition, 130 cleft sentences(註一), 130.5 other structures
"....
All (that), and expressions with thing, can be used in cleft sentences.
All I want is a home somewhere......"
All you need is love.
All (that) I did was (to) touch the window, and it broke.
The only thing I remember is a terrible pain in my head.
.......
(註一) cleft是"divided"的意思，這用法的出現是為了用文字來凸顯/強化某個特定的字。我們一般在說話時可以用語氣上加重某個字的方式來強調那個字的意思，比如:
我在看電視. (普通語氣/速度的念)
我在看*電視* (粗體字是大聲念出來的部份)
這兩句聽起來的感覺會不一樣。但是在文字書寫上就沒辦法這樣子做(除非像我一樣在後面刻意加個注解補充)。cleft sentence就是在這情況下產生的，它是先用一小段話來作前導(也可以是在後頭)，告訴讀者「注意喔，接下來的字(或是之前的字)就是我要強調的喔」，之後所接的才是真正要講的，最後還可以再用個類似關係子句般的子句把主要強調的字補充完所要說的意思。
比如：
- Mary kept a pig in the garden shed. 這句話很平常，沒什麼特別，但是如果我要強調是"Mary"而不是其他人的話---
Mary was the person who kept a pig in the garden shed. 這是把要強調的Mary放在前頭的用法，然後 the person who 就是告知讀者前頭的主詞就是我要強調的。
The person who kept a pig in the garden shed was Mary. 這是把要強調的Mary放在最後的寫法。
"cleft sentences"可以用的基本句型很多，前頭的All (that) .... 也是其中一個。
以原來的問題來說:
All I could do was (go to bed) and (hope that I felt better the next day).
這整句是要強調兩件事(括弧部份)，而不是要強調開頭當作主詞的All。
其它資料:
http://grammar.ccc.commnet.edu/GRAMMAR/sentences.htm
"The cleft sentence .... (usually who, which, or that)"
-----------------------------
Comprehensive Grammar of the English Language
Quirk et al
18.28 : page 1386 - 1387
(Cleft sentences)
"18.28 The second clause in a cleft sentence is obviously similar in structure to a restrive relative clause; .... Examples above show that pronouns used in relative clauses (who, that, 'zero' pronoun) are also used to introduce cleft sentences.......
"There are differences from relative clauses, however, in that the wh-forms are rare in cleft sentences in comparison with that and zero. Although whose is allowed in cleft sentences (It's Uncle Bill whose address I lost), whom and which are only marginally possible, and it is virtually impossible to use whom or which preceded by a preposition. ......." --> 謎之聲: What happened to who?
(Note: There's more to the restriction of using wh-forms)
----------------------------
"Practical English Usage" by M.S., 3rd edition
131 cleft sentences (2): .....
"... are usually joined to the relative clause by that. ....... Who is possible ... when a personal subject is emphasised."
