A question..

Hello there,

I have a few questions and I do not know where I should get feedback from. Normally, I don't have a tendency to use others as a reference, however, in this case, I find it to be necessary. Before I begin, I should state that I might come off as a little blunt and I do apologize if that is so, I mean no offense.

My question:
How would one know if they have a mental illness?
or to rephrase; how would one distinguish their traits from their mental illness/personality disorder?

A little backstory.
My parents are extremely conservative, and I have never been checked for any disorders of sorts. I am 23 and it has come to my attention recently that I lack the ability to connect with my environment and with the emotions of others. Alongside that I seem to be losing touch with reality quite a bit, easily confusing my overly vivid thoughts with memories, believing them to be true, confusing myself about past events, emotionally fragmented and physically numb. Some would say I'm in a state of.. liminality. Tied in with that, I lack the ability to express myself. However, since I've experienced this from a young age, I've considered this to be a part of who I am, defining traits. They were 'normal' to be and I could not see how I was perceived to be a problem with those traits.

Up until recently, upon being exposed to the social world. My traits have become, 'abnormal'. A problem, and unusual. I have never explained it to my parents, and they never noticed. I do not know how to progress other than to keep to myself and to put a stop to interaction. It is a little overwhelming, but I am curious about the 'problem' and how one should view this issue. I really would appreciate insight.

I should also state that I prefer to get a response to my question above, instead of advice on the situation I described below it.

When the condition seriously interferes with normal functioning, it can be considered a mental illness. Prolonged and excessive fatigue, for example, is not normal.

On your own, you can't. Seriously, even qualified psychologists don't diagnose themselves, because they're not objective. Only another, qualified person can do that.
You might get some clues, when some of your behaviors start to interfere with your own enjoyment of life, but you can never be sure on your own. I think the way you phrased the question is also a manifestation of the problems you might have.