I want to have a threesome with my husband and another girl, but I feel really nervous

This is such a good question, and there's lots to explore. This "empty" feeling - if you were to stay with it, not trying to change it, just trying to understand it better - what does it tell you? Are there thoughts that come with it? Fears/concerns? How does it feel in your body? Is that a familiar feeling? When did you feel it before? 

I'd want to get clear about your needs/wants/desires - and also your values and beliefs around intimacy and sex and marriage.  Are your desires in alignment with your values? Maybe there are some old values that need to be re-examined? Are your heart, mind and body in alignment when it comes to this desire? We live in a society that's pretty judgmental when it comes to stuff like this, even though threesomes are one of the most common fantasies.  

Is it possible that this empty feelings is the excitement and fear, overlapped, like when someone is trying zip-lining for the first time? 

Are there other feelings underneath the emptiness? Shame? Pressure? Excitement and anticipation sometimes brings a feeling of shame or guilt - as if "is this ok for me to feel that?", or "is this normal?", or even "what's wrong with me for wanting this?". There's nothing wrong, and it IS ok to feel all of your feelings.  

Is there a conversation with your husband that needs to happen before you continue exploring? Do you need to find reassurance, closeness,  to talk about boundaries, to make sure it truly works for both of you. Do you talk about the first experience - what worked about that, what didn't, what would make it even better, what are you hoping to experience next time? How does it feel to talk about it? 

Keep moving forward, slowly, while paying attention to your body, your heart, and your thoughts and feelings. Keep talking about it. Journal if you like it.  You will find your answers.