The fear of the downs of life

You always hear people saying how you should always worry when you are truely happy because that means a big drop in your life will happen soon where you will become really depressed. It's like a rollercoaster...once you reach the highest peak, there will be a sharp drop. And that the higher the peak is, the bigger of a drop you'll have. From personal experience I do see that happening alot but I hate to think that oh my God I'm happy now so I really need to be careful because there will be a huge drop soon! Living life like that would be too depressing...when you are happy you are expecting the worst, and when you are in the worst you are in no mood to think of the happy. But for some reason, a few days ago, I felt truely happy for a few mins...and I was very conscious of it too...and for the first time I was actually scared to lose that feeling which is not me at all! I've always been quite moody...feeling hyper one sec, then really quiet the next...happy then a little sad...but I knew that that was how life was, up and down all the time...but for some reason this time I was so scared of the down. I still am scared of the down...my downs usually mean I switch off for a while...put a distance between me and the people around me and I do it without realizing it too which makes it more difficult to control. And I am worried of falling deep into that this time around for some reason. I'm dying to know why.I'm very scared...