You had a normal life, yet now you feel split.
You can't stand me, yet I still harass you.
You just want me to go away, yet I rarely leave you alone.
You explain what I'm doing to others, yet they don't believe you.
Most have heard about me, yet never actually seen me.
You feel so alone, yet you're not really the only one I'm harassing.
Who Am I?
Based on fontastico's answer, I think the answer is
Schizophrenia
You had a normal life, yet now you feel split.
Schizophrenia is a mental disorder with no single cause, it can happen to anyone. One of the symptoms is hallucinations, causing "other people" to inhabit within you.
You can't stand me, yet I still harass you.
You just want me to go away, yet I rarely leave you alone.
Generally, people dislike schizophrenia, as it is quite the destructive disorder for the person who has it, and people around. No cure has yet been found.
You explain what I'm doing to others, yet they don't believe you.
It's hard to believe what hallucinations are doing to someone when you're not prone to them yourself.
Most have heard about me, yet never actually seen me.
Schizophrenia is a fairly well-known mental disorder, yet there is no real way to "see" schizophrenia itself. It's not really a tangible illness like a virus.
You feel so alone, yet you're not really the only one I'm harassing.
Schizophrenia can cause ties to be broken and loneliness. About 24 million people on Earth are affected by such a disorder, making it a serious issue, and not an isolated case.
Title:
The last word is an anagram of the word "DISORDER"
Could you be...
A depression?
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.
Bonus:
I thought of this answer because the title has RIDDERSO and, unscrambled, its a DISORDER. Maybe referring to mental disorders like depression.
