Could you be...
You had a normal life, yet now you feel split.
When depression strikes hard the way it strikes it breaks you apart, normally. I understand there are a lot of manifestations that depression have, but normally the most common one is to feel bad, split, like you do not exist but at the same time, exist.
You can't stand me, yet I still harass you.
Depression is hated, but unfortunately, even trying to avoid it, it harasses us.
You just want me to go away, yet I rarely leave you alone.
Same as before; once depression strikes, for example, with anxiety, we want it to go away soon, but in the end depression will always go with us
You explain what I'm doing to others, yet they don't believe you.
This is the part i struggle to explain actually. I think it is because depression is truly a disease but normally the common response when we share how we feel is something like "hey, cheer up", leading to think that they do not really believe you.
Most have heard about me, yet never actually seen me.
Depression is a common topic, but it's invisible because it is a mental disease.
You feel so alone, yet you're not really the only one I'm harassing.
Depression itself is a common disease nowadays and makes us think we're alone, but in the end there are many people around the world that suffer it.
I thought of this answer because the title has RIDDERSO and, unscrambled, its a DISORDER. Maybe referring to mental disorders like depression.