Replying to I'm sad during good things.

ive noticed that something good is about to happen i get really sad because all i can think about is that it will be over. for example, in 2 days im going to maine, which will be really fun, but i cant stop thinking about it ending before i know it and me getting really sad about it. or lets say im going to the moon, (which quite possibly would be the best thing that could possibly happen to anyone) all i would be able to think about during planning, the trip there, and when im actually on the moon is that it will all come to an end. another example is this coming ski season. im looking forward to it soooo much, but i again cant imagine it ending, and what i will do then. (although this is a less extreme example because i realize i have many, many more ski seasons to come)

so no matter whats going on thats good, (excluding good things that will happen to me so many times that i grow accustomed to them) i cant feel any optimism or happiness about it. it really sucks.

so if any of you guys have or have had this problem, let me know of ways to make myself be happy about stuff, or ways to block the feelings of doom.