Why can't I grieve for a lost friend?

The question posed, "Why can't I grieve for a lost loved one?" is a very important question for many individuals, for various reasons.   First, what is grieving?  Grief is feeling sorrow, deeply.  Mourning is the expression of deep sorrow for someone who has died, typically involving ways to demonstrate the mourning like wearing black clothing.   
Experiencing the feeling of emptiness could be a form of how you are grieving.  As written in the question, there was a lot of time, resourced, and emotional experiences committed to the friend.  These experiences ranged from the mundane to heroic, which is extraordinary.  The time spent, the processing of schedules, the dependency, etc. required all of you and now that the friend has died, the focus, the time, the energy, the planning, and the interdependence is gone, possibly resulting in the empty feeling.   

It is a strong possibility that you are grieving and not accepting that you are grieving.   There may be a preconceived notion on what mourning/ grieving should look like, but there is not just one way to grieve.  Give yourself a break and feel the sorrow and accept that you can move on, in honor and not forget your friend.   There is not a prescribed way to grieve.   

There are studies and theories concerning grief and one of them are the 5 stages of Grief.  These are denial, anger, bargaining, depression and acceptance. If you are experiencing any of these emotions following a loss, it may help to know that your reaction is natural and that you’ll heal in time. However, not everyone who grieves goes through all of these stages—and that’s okay. Contrary to popular belief, you do not have to go through each stage in order to heal. In fact, some people resolve their grief without going through any of these stages. And if you do go through these stages of grief, you probably won’t experience them in a neat, sequential order, so don’t worry about what you “should” be feeling or which stage you’re supposed to be in. (https://www.helpguide.org/articles/grief/coping-with-grief-and-loss.htm). 

Let me encourage you that you can choose how to honor your friend's memory to help you to accept that you have accepted the death.  Also, keep in mind that you have to replace your purpose with the absence of duty.  Invest in yourself, being healthier, activate a hobby or interest and do not allow automatic negative thoughts to influence you into believing that something is wrong if you do not act or think a certain way.  You do not have to cry to grieve.  

I hope that you embrace the hero that you are and the great friend that you demonstrated to your friend.   

Please reach out to a professional, friend, clergy if you need any further support.