Rainer Maria Rilke's Letters to a Young Poet gave me great comfort and encouragement when I was 17. It's been years since I've reread his collection of letters, but I find them to be a source of inspiration still. Here's a passage on solitude that I find especially timeless.

...there is but one solitude, and that is great, and not easy to bear, and to almost everybody come hours when they would gladly exchange it for any sort of intercourse, however banal and cheap, for the semblance of some slight accord with the first comer, with the unworthiest...But perhaps those are the very hours when solitude grows; for its growing is painful as the growing of boys and sad as the beginning of springtimes. But that must not mislead you. The necessary thing is after all but this: solitude, great inner solitude. Going-into-oneself and for hours meeting no one—this one must be able to attain. To be solitary, the way one was solitary as a child, when the grownups went around involved with things that seemed important and big because they themselves looked so busy and because one comprehended nothing of their doings.