How do I tell my Extroverted partner that I need alone time?

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So Im more Introverted and my boyfriend, is "extremely extroverted" (his words). He always likes being around people, while I like my solitude.
We just spent the last month together travelling, and just got home. I got upset at something that happened here and needed some alone time, and he offered to come over. When I declined him and told him that I would like to be alone, he took that as him doing something wrong. How do I express to him that it isn't him?
How do I tell my Extroverted partner that I need alone time?
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