I see this as being about a relationship. Now that he is alone (Now that I lost what I thought I had gained) he is full of hate and anger toward the other person (Sourness and fear is all I taste). Looking back he can see that he wasted his love and feelings on this person (regrets ever being in the relationship probably) -> He goes into more detail about the anger and how he was left without closure (Look into the fires you left behind. Smoking ashes sear my mind)
I see this as being about a relationship. Now that he is alone (Now that I lost what I thought I had gained) he is full of hate and anger toward the other person (Sourness and fear is all I taste). Looking back he can see that he wasted his love and feelings on this person (regrets ever being in the relationship probably) -> He goes into more detail about the anger and how he was left without closure (Look into the fires you left behind. Smoking ashes sear my mind)