Help, I need input!!

So my husband is really close with his bestfriend (Steven) they hang out a lot. When Steven got married he asked my husband to be his groomsmen and his fiancé (Kelly) asked me to be her bridesmaid, I felt obligated to say yes. Because of the wedding I ended up spending a lot of time with Kelly and cultivated a friendship, but before then me and Kelly were not friends we only started hanging out because of our husbands. I like Kelly but do not feel I have enough in common with her to consider her my best friend yet she has called me one of her closest friends... Now every time my husband and Steven hang out she wants me to hangout with her. I’m very introverted and enjoy alone time, today my husband is going over to Stevens house, Kelly messaged me asking to come over to my house since they guys are hanging out at her house, I was looking forward to my alone time. I told her we could hang out another time but now i feel bad. We did hang out last week, we went to see a movie. I don’t like being around people to much and I don’t want to be mean. I don’t mind hanging out with her every once in a while or with our mutual friends I just don’t think we need to hang out every time our husbands do and I don’t think we need to be bestfriends just because our husbands are. But how do you go about saying that when she considers you one of her bestfriends? I feel kinda of mean but it’s not personal I just am not the kind of person that likes to hang out with others every weekend, please help any input is appreciated thank you