I know a very similar fear and struggle to deal with it myself
pay attention to this bit that you wrote
Originally Posted By: Little
happiness with a man I so deeply and honestly love?
In your sentence you said 'a man' - when you said that did you mean BF or did you mean what it says 'a man'. the second one is better as it you are not investing all your happiness in one particular person.
what do you want from life? make a list and then consider what parts of it could ONLY be fulfilled with your BF - im hoping the answer is none. on mine the things that can only be fulfilled by my W are 'to only marry once and have a loving and successful life together' and 'to raise my kids with their mother'
one of the things i read in book was to think about all the parts of your life as compartments and then see how well they are going independtly. if the only compartment you have is your R then the loss or hurt in this hurts the whole you but if the other parts are all good then the R difficulties matter less.