Why is coming out so difficult

I was planning on just never telling anyone ever but life throws curveballs at you, I have a boyfriend now and its really hard to not talk about him with friends or family because I really want to, hes such an important and big part of my life. Everytime I script a coming out scenario in my head that I want to say to someone, I cant get it out. The only person that knows is a lesbian co worker of mine, and I even struggled to tell her.

Its just been a secret for so long that my brain doesnt let it happen, I was gonna tell my best friend and literally froze mid sentence. Im not sure how to get over this. I dont have anyone in my life that I care to keep in my life that wouldnt at least eventually accept it, I wouldnt be in any danger. Why do I struggle so hard to say it?