I think from my own experience, introverts like me and you, need more time alone for whatever reason, that's just the way we are. If you need more time to think about this relationship, tell your friend you need some time to think and the reason why, being an introvert, and she would have to understand if she wants to get married or not , you need the time for yourself and it's not because of her. Hopefully she will understand. She will have to know that before she commits to marriage as she will have to deal with this problem after marriage too. I do hope she understands!!