Why is のにと used here?

Question

　あんなに思い合ったのにと悲しくて、　何度も約束したのにと悔しくて、　込み上げてきた涙で視界がじわりと滲んだ。

I understand the use of "のに" as "despite" however what function does と add in this sentence?

Answer 1

Think of this と as the quotative と. And for each emotion: 悲しい,悔しい the と is marking what the feelings of sadness and frustration are over.

It would seem a couple has broken up and the one is very sad because she/he thought they loved each so much; she/he is frustrated thinking over all the promises made [to one another supposedly].

You can think of it a bit as though the word 思う or some similar verb showing rumination has been omitted. (Look at the previous paragraph... that's exactly what I did when summarizing the situation as I see it).

あんなに思い合ったのにと[思って]悲しくて

Oh, but [I thought] we loved one another so much

Here "Oh, but I thought" being a rough translation of what's being expressed by のにと. The "oh, but" here corresponding roughly to のに.

In a bit more literal translation

Oh but we loved one another so much... I'm so sad. Oh but all the promises we made... I'm so frustrated [or, it's so regrettable].

This is a common way of abbreviating what one is saying. So, when you see と in circumstances like this, try to think of a verb that might suitably follow.

Note: If you're starting off learning Japanese, that might be a little hard to know how to guess. Then try to look at the broader context in which the sentence appears. The overall sentiments should help make sense of what's being expressed.

Answer 2

「あんなに思い合ったのに」と悲しくて 「何度も約束したのに」と悔しくて

「あんなに思い合ったのに」and 「何度も約束したのに」are lines.

In more detail, they are as follows.

「あんなに思い合ったのに」と思って悲しくなり

「何度も約束したのに」と思って悔しくなり