Why am I getting so jittery ( a sense of impending doom) about school again? It's going to be my senior year?

For 3 years I've worked my *** off and now finally it's senior year where everyone says there is minimal work and we just get to chill mostly. Of course there are college apps but that's only for a few months. Anyways, today I just realize school opens in a week and I have to go back and I'm feeling all the feelings I felt back during my last 3 years before school started. Is it just my mind that's gotten used to the terrors of high school or is it something else? How can I make it go away - because truly, this worrying is for nothing. It's meaningless!