My head feels like it will explode due to resentment accrued over the years

Are you at least mid life or is this some 19 year old who is JUST starting to learn how bad life sucks?

Either way -
Yes, resentment does tend to build over the years. Like tonight on Xmas eve, instead of having been invited to any sort of family function, my room mate and I are sitting at the house trying to have our own miniature celebration. Gives me time to reflect on how i have pretty much always been treated like an unwanted "extra".

Sadly, although it should not be the norm, you're situation is a common one. I do, however, think that such hardships would quite naturally drive a person crazy. I think your feelings are normal. I think it's normal to have resentments against family for bad things that have happened.

Is there a history of substance abuse in your family? I assume there has been a lot of abuse, and neglect?

Please find yourself a good therapist to get rid of all that resentment: it's only hurting you