No, you're not selfish. Too often it is assumed that depression is always situational. While that may be the case for some, it is far from always the case. Depression can't quite be explained. As it seems that everyone experiences it differently and for different reasons.

As for how to make yourself your regular self again, you've already achieved the first step. Realizing that something is wrong. Now, the best thing I can recommend you do is reach out to someone. Have you tried to speak to a parent provided they'll listen? Have you tried speaking to your school counselor? It might sound scary but it does help to talk. It not only helps to find help (such as therapy if it's a possibility) but it helps because sometimes talking is what we need to do most.