Of course you can grieve & mourn. In fact, I think you must, because you obviously need to.
I had a termination a few years ago, and I would say that you go through a grieving process of sorts. I did what I knew was right & I have no regrets. But it was still a hard process to go through! And same as you, I am pro choice etc. That doesn't mean it doesn't hurt, didn't bother me to have to do it. Some people think that being pro choice means we enjoy hearing about/having.encouraging women to have terminations, but that obviously isn't true. We just believe that a woman is allowed to decide if it is her best option. And it also means that we can support those who do take that option!
I have no idea if what I am trying to say is coming out as I want it to. In short, just because you are pro choice, doesn't mean that you can't grieve for your baby/pregnancy. Being pro choice doesn't negate those feelings.
I was told (by a friend) that perhaps explaining to the baby I didn't have, why I did it. And to say goodbye. Now, I know that may sound daft, especially to those that don't believe in all that stuff, but it helped me. Maybe it was the saying it out loud, maybe it was 'talking' to my child that never was,...I don't know, but in some way it helped.
No one has any right to tell you what to feel, so if you feel grief, grieve. Holding onto such a strong emotion isn't going to help you! If you don't want your little lad to see you upset, when he is asleep, have some private time & do what you need to do to get you through this.