Can’t shake the feeling !!

This is the third time I’ve cried my eyes out over this. It usually happens when me and my boyfriend lay down to sleep. I’ll feel like I’m running out of time with him. I understand that I’m technically not but It bothers me so much. I’ll cry and cling to him for almost an hour. Our child will change our lives forever obviously. I’m without a doubt excited for him/her but I get these random surges of this sadness. I don’t understand why. Any ideas or advice?