These feelings are extremely common among depressed people.
Right in the first paragraph of the description of clinical depression on Wikipedia it says "Depressed people may be preoccupied with, or ruminate over, thoughts and feelings of worthlessness, inappropriate guilt or regret, helplessness, hopelessness, and self-hatred". But what is said after that is very important: "These feelings are not just about being depressed, they may be delusional".
What this means is that when you're depressed, your whole view of yourself changes dramatically, you judge yourself very differently and far more harshly than other people judge you. You invariably judge yourself as inferior to almost all other people and constantly berate yourself for being what you perceive as "sub-standard". I know, I do it almost incessantly myself, which is why I have great difficulty forming relationships with people; it makes you despise yourself, it makes you presume that everyone else despises you, and that therefore you can't trust anyone else OR yourself.
When it comes to thinking over your past, just remember: "It's what you do last that counts".