赞同蒋老师的解答。补充一点，《韦氏词典》认为这种用法的as为副词，通常用在介词或分词前。

老师们的回答都有道理。

我也说说自己的看法：

问题句中as between和下面的相似：

affinity：relationship by marriage (as between a husband and his wife's blood relatives) — distinguished from consanguinity

cohesion：the union of separate plant parts or organs — used chiefly of union between parts of different floral whorls (as between sepals and carpels); compare

Alimony may also be granted without a divorce, as between legally separated persons. （赡养费亦可付给并未离婚的双方中的一方，如在合法分居时）

as可看作介词，意思是“如同”。句中as和between是自由组合关系，介词不限于between，再如：The 'h' in honest is silent, as in 'hour'.

所以，

as>[between A and B] , between A and B作as的宾语。

And as between apathy and pain she had made her choice. = And she had made her choice, as between apathy and pain.

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曹老师查的韦氏：《韦氏词典》认为这种用法的as为副词，通常用在介词或分词前。

我也查了韦氏，说法不一样：

Webster's Third New International Dictionary, Unabridged

IV. preposition
Etymology: as (I) 
1. : after the manner of : the same as : like
 < had seen strong men become as weaklings when they were faced with … being shipwrecked — H.A.Chippendale >
 < his face was as a mask of gauze through which nothing was quite clearly visible — Max Beerbohm >
2. 
 a. : in the character, role, function, capacity, condition, or sense of
  < more interested in … attitudes as attitudes than he is in their definition and embodiment in aesthetic forms — Mark Schorer >
  < eager for power as power >
  < his appearance as Hamlet >
  < his appointment as instructor >
 b. : in a way or of a nature constituting
  < he comes home at six as a rule >
  < as a result of the trip he was exhausted >
 c. : for consideration or considered in a specified form or relation — usually used before prepositions and participles (as against, between, distinguished, and opposed)
  < his argument as against yours >
  < my opinion as distinguished from theirs >

fleshly, fleshy. The distinction is much the same as between earthly & earthy.

这里 = as + between 的自由组合

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但题主的句子里，未必是上面的那种含义

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She was honest enough to admit that life had failed her. And as between apathy and pain，she had made her choice.

前句说，她承认生活已经使她失望，接下来说，_____ 麻木与痛苦，她已经做出了她的选择.

看不出来在横线处，为什么要填入“正如 ... 之间”之类的字眼？

我倒是觉得as between = in terms of, 即：就 麻木与痛苦 两者而言, 她已经选择了好了（其中一个，也许）。但as between 的确又能体现出一点 “两极”的味道：要么apathy  <-----> 要么 pain. 说不定是作者临时造的词？

不是固定词组，字典里没有。

And as between apathy and pain she had made her choice.

等于

She was honest enough to admit that life had failed her. And （it is just）as she had made her choice between apathy and pain.

正如他在between apathy and pain作出了选择那样。补充说明前面一句话，“承认...就如她曾作出的...选择”。