Dear Tineoidea,

My impression is that your girlfriend was/is confused about herself (perhaps even her sexual identity?), and therefore easily influenced by others. This is what suggests her confusion:

Eventually, and after a few weeks, she broke up with me and wasn’t able to give any coherent reason as to why. It was always a “I don’t know myself”, “it’s how thing are.

Also, she was very unbalanced, sometimes very affectionate towards you, at other times abrasive and angry:

I should also mention that she’s been extremely unbalanced during the whole ordeal, going from affectionate and rational to abrasive and angry.

This too shows a very deep internal conflict:

What surfaced during this interaction is that she indeed still loves me, misses me a lot and doesn’t feel romantic love when it comes to my former friend.
Yet she still wants to try things with him and absolutely doesn’t want to give us two a chance, she wants to move on.

She loves you, and yet doesn’t want to have anything to do with you? She feels no romantic love for your male friend, and yet she wants to “try things with him”?

It might suggest confusion about her sexual identity. Do you think it’s possible that she feels guilty for being attracted to the same sex, and he is feeding this guilt? And she chose him to prove to herself that she might be able to like men too? This is just a guess, it could be totally wrong, I am just exploring the reasons for her 180-degrees turn.