P.M.
A quick punch line crossed my mind: your little guy has found a birth control system to prevent any more little interlopers in his family!
But seriously, it's wonderful that you feel for his night fears (very common at his age) and his sadness at being moved out of your bed. This empathy is a wonderful thing. AND it shouldn't keep you from doing what is "most best" for all of you. If this is becoming a chronic situation, you will probably do all of you a favor by finding gentle steps to break the cycle.
You can google around and find a lot on "children, fear of dark" or other phrases that describe your situation. Lots of great tips that work for other parents.
As for dealing with your own sadness, I've discovered that other people don't technically "make me" feel bad, sad, glad…. Those feelings are my own, and can't be manipulated by others so easily once I realize it. When I learn to take care of my own emotions (which means learning that they arise from my own personality, training, etc.), I begin to teach my children that they can do the same.
It sometimes helps with my 4yo grandboy, when he makes a statement like, "If you don't let me do x, then I'll be sad all day," to acknowledge with sympathy that he would like x. Then I go on to explain, gently, why he won't get x (perhaps only for this time). I tell him that I would be sad for a little while, too, but that it's much more fun to be happy, so I would get happy as soon as I could. This coaching can help him begin to notice choices he can make for himself. A wonderful life skill!