Hello again, when I read your post the first time I knew exactly what you were talking about because I have gone through, and still do to some degree, the very same thing. I realized this a few years ago and began to try to find why and exactly what I was feeling. For me I found that it is unavoidable at some level, but I have learned to lessen it a few ways.
Firstly, realizing that the sadness is not needed at all, especially when you just had a great time...at least for me, has a bit of logical sense and also makes you think of the enjoyment of the trip. Another idea that helped me is somewhat related to the first, being that when I notice the sadness coming on as the occasion comes to an end...I realize what it is and begin to focus on only the good parts, the people, family, friends...and you can begin to sculpt the appropriate mood for the happy occasion. As you even said, "there will be more vacations and I will spend time with those friends again", so each time you work at correcting your mood you will get better at realizing that fact sooner and sooner. Also you can, instead of falling into the sadness (feeling of encompassing loss from leaving the great situation), you can start thinking about and planning the next great occasion. This is an awesome way to pull your thoughts in a positive, happy direction and also it is constructive in that you are already planning the next fun time :). Now these are just the things that worked for me, and everyone is different, so I just hope that you can use some bit of them to create your own personal coping method. It IS possible for sure, I know that from my own experience with exactly this problem, you can do it for sure ;-). Eventually you will have so much more control over your feelings in the situations (being that even I still feel a bit of sadness on the same occasions) and be able to handle them much better. Keep on posting and let us know how you are. I read new posts and reply at least every two days so if you ever post I will definitely read and reply with my two cents on the situation if I have it. I also have my email available on my profile, so feel free to hit me up anytime if you want to talk. For that matter anyone that wants, feel free, hehe I know that sometimes its easier in a one on one situation...but for the grand majority I'd say you found the right place :)