Help. How to get into a top-tier graduate school (Oxford, Harvard, etc.)?

Not sure why this is is Marriage and Divorce, but...assuming you're going for an academic degree, grad schools care about your academic achievements: grades, standardized tests, and published research. Extracurriculars and work experience are nice, but unless they're directly related to the former in some way, they're not a major factor. And yes, you may be at a disadvantage compared to candidates who didn't have your struggles from the start (or who are coming out of more prestigious undergrad programs), but as long as you can argue that your newer transcripts reflect who you really are and what you're capable of, that'll be taken into account. Besides, if this is what you want, then why shouldn't you try?