Really Just A Bunch Of Definitions . . .

Isolate:  To set apart or cut off from others.
Who are these "others" of whom you speak?

Solitude: Aloneness; state of being alone or solitary; by oneself.
Do you see the difference between the two concepts?  Being by oneself can be a good thing and it can be a bad thing.  I have to be careful with alone time because I have a bit of the introvert in me.

Introvert: (Of a person) Who focuses primarily on their own mind, feelings, or affairs.
Kind of sounds like a selfish jerk, the introvert does.

Selfish: Having one's own self-interest as the standard for decision making.
So some differentiation between the introvert and the individual who is purely selfish, but not as much as I'd like to see.  I view introversion as a character trait that is baked in from the start and selfishness as a learned behavior. Sometimes I justify my behavior by claiming Originality instead of admitting Learnedness.

I'm assuming you can have selfish introverts.

I have believed for the longest time that I'm a pure introvert because I long to get away from other people so much of the time or that I like to limit the time I spend with your average joe to brief, staccato incidents where I'm in control of the process.  I think I'm just more of a jerk vis-a-vis other people than an introvert.  I think I mask disdain by claiming an ingrown character trait.

Anyway . . . the point is that I have always had to keep an eye on the amount of time I spend by myself.  So many of the things that I like to do are alone things - reading, writing, meditating, the kinds of exercise I favor, especially hiking and swimming.  

All of a sudden I'm spending great chunks of time by myself, inevitably thinking about myself.  I'm in the midst of one of those runs right now and it isn't doing me any good.  My thinking has gotten inbred.  My thinking has gotten unsound.