Lost libido in relationship?

I feel terrible, but I’ve been in a relationship for 4 years with the same guy. Over the past 6 months or so I have noticed that I don’t enjoy having sex with him anymore, I don’t even really enjoy making out and I have no idea why. I have literally tried everything to reignite it, I still feel “in the mood” but I just don’t have the urge to do anything with him. I feel like a horrible girlfriend but I really don’t mean to be like this, I know sex isn’t everything but it’s still a really crappy situation. Has anyone else experienced this or have any advice?