letting go

Feeling really sad about the prospect of letting go of what is dear to my heart. Knowing and not knowing.... knowing that I am in a moment of change, seeing doors open and new horizons - literally and figuratively.   Not knowing for sure if this is a door i need to close or is it a moment to stay with till the lesson comes? Knowing that if I don't learn the lesson, it will come around again. What to let go of is the real question.  I must let go, I just need to understand of what? Of everything or part or outcomes?