The meanings are different.
"She considered me her boyfriend," means that, at that time, she felt I was her boyfriend. This is also the meaning of, "She considered me to be her boyfriend."
"She considered me as her boyfriend," means she considered the possibility of choosing me to be her boyfriend. This is the same construction as, "The company considered you as vice president, but decided that she was better qualified."
I think that confusion of the meanings of the two sentences may be partly due to the following:
"She considered me her boyfriend." = "She thought of me as her boyfriend."
In other words, the confusion comes from subconsciously substituting "thought of" for "considered." The two are synonyms, but "thought of" requires "as" in that construction.
"She considered me as her boyfriend." = "She thought about me being her boyfriend."
That would mean the same. I don't think there is any ambiguity either way. But I guess that might clarify the meaning for a non-native listener:
- Me: I consider you a friend.
- NNL: Do you mean that you think we could be friends?
- Me: No. I consider you to be a friend already. We are friends, right?
No. Adding an 'a' would render a different, strange meaning: she considered me to her boyfriend. The object is yo (represented by 'me') not su novio. 'Su novio' is just extra details when it comes to deciding subjects and objects. I don't remember, I think they call that 'compliment' when they teach Spanish to English speakers.
In cases like this, the personal a would only be used if you use a redundant 'a mí', although I don't know why you'd say it that way. When you have a reflexive type of construction like this, the personal a is not needed unless clarification is given as to who the direct object is.
That line makes me think of the song 'La légion marche vers le front' (the legion marches to the front) from the French Foreign Legion (don't know if links are allowed, but you can find it on YouTube).
It has quite a dark text and an even darker history (it was adapted from an SS song), so it would seem that fits your reference.
Novio: boyfriend or groom. (Lover: amante) This is why it's important to learn about culture while learning another language. In many places there isn't even 'engagement', a couple dates and then decides to get married and they just do it. Other places both of the pareja get rings when they decide they want to get married and then at the ceremony the rings switch hands. There are a lot of variations, but language reflects culture, so we can't think of things with regards to our own culture and experiences.
In the Netherlands there is a different interpretation that follows roughly the generational divide: people younger than 30 or so interpret engagement as the period between mariage proposal and the mariage itself: you're getting married and between now and a few months you will be picking a date. Older people tend to see engagement as a more formal thing, for which you can have a party and exchange rings if you like. Such an engagement might be several years before marriage and marriage is not a 99% certain outcome. Before engagement you might say 'alles kan kapot' (everything can break, i.e. you could try to break them up to try having a relationship yourself with one of them), while during that engagement you really really can't say that.
why is this in the preterite? boyfriend/girlfriend relationships are often longer term than a moment so why isn't this translated in the imperfecto since "she *used to consider me her boyfriend" is better english when we are aware that we have to make that distinction of "how things used to be" versus finished past tense? is this conjugation supposed to show that she's over him?