Sex drive is Low

Okay so I've been with my boyfriend for 4 years now... he's amazing , I love him... sex was always amazing ! And I mean amazing !!!! But now... it's not the same. I'm physically and mentally attracted to my boyfriend but when it comes to sex I lose interest? 
Is that normal ? How can I fix this? Like how can I re spark that up again ?? 
Ladies please ! I'm desperate because I'm hurting the man I love .