I've always felt very similar, OP. Part of the reason that when I was first typed as INFJ, I really resonated with that.
I used to think I was extroverted but when I really considered the meaning of those words, it made so much sense. Especially the concept of "energy" distribution because I know after a fun social day with my friends (even if we exerted very little physical force), I am pretty exhausted. That fun kind of exhaustion. I love that feeling but it's obviously not realistic all the time. I much prefer to be with my own thoughts, ideas, daydreams and such.
Well, it's more that I can't imagine being ONLY social and hanging out all the time. During my Spring break, I hung out with my best friend almost every day but time was running out for me to have a day all to myself and it was driving me a little nuts, I felt off. (I bet it was the same for her) so I do NEED it, it's not JUST that I like it.
It's also not just about MY thoughts or all about ME. That is something I think a lot of INFJs have to recognize. But on the whole, it's simply not that selfish. Self-introspection can be very important and is a legitimate way to use your time in my opinion. But, definitely in this world, you must think of and deal with others and in that time of deep thought, the more you incorporate others as well, the better.
Introverts always tend to be on the internet a lot, for obvious reasons. Ever since I figured out how to go online, I've been an internet junkie. (Explains very obviously why on MBTI forums the INTP, INFJ, INTJ and INFP people post the most, OVERWHELMINGLY).
As for an extreme preference or need rather than a more slight one, I'd say for me that fluxuates. When I am busy, I tend to want to be alone less because I know I will not get work done (ironically) without a little external stimulation and encouragement, maybe even help with what I need to do.
When things are going good in my life, I tend to be alone more. When things are bad, it can sometimes be dangerous to be inside myself so much. This is a recent development though, in realizing more healthy ways of dealing with depression and pain.
Gladly, I have the right people in my life now (as opposed to when I initially had depression) that I know will support me in my thoughts and make an effort to understand me.
It's hard for a lot of introverts I believe to open up, given the frequency they try to solve their problems within themselves.