How do you go from being a best friend to lovers

Dasha
Me and my best friend have been friends for 10 years we did everything together I have even hooked him up with a couple of people we even watched porn together and of course made fun of the people.Anyways we have been together for 8 months now ttc engaged but sometimes I forget and I have been looking at girls for him and then I'm like oh yeah you are mine he just laughs and says I'm all he wants but I forget has anybody else ever fell for they best friend and sometimes fall back in the best friend phase I don't mean to I don't know I'm so confused I know I am in love with him but other times I wanna keep my best friend that rags with me please help... don't be mean please but I wanna hear similar stories maybe one of you lovely ladies can help me to transition like can I be both or do I have to decide???