The main point is what to do about "inoportunidades". This doesn't mean hopelessness, or inconvenience, though I do think that the whole phrase conveys the idea of hopelessness. It means "inopportunenesses", which really amounts to the idea of bad timing: a succession of things happening (or perhaps being said) at the wrong time. I think it really refers to a relationship in which nothing runs smoothly. That's how I got to "awkwardness". In fact, both "inopportuneness" and "untimeliness" are listed in the thesaurus I consulted under "awkwardness". The relationship has just become constantly awkward.

"Caímos en un vacío", for me, expresses falling gradually and almost unconsciously; something close to slipping. I think "lapse" could work, but "fell" would be OK. But I feel that "void" and "abyss" are too dramatic for "vacío". To me, what this expresses is just a state in which there's no joy, no pleasure. It's empty in the sense of being bleak.