Have you ever been so happy before that you can't help but feel scared, because you know a "crash" is inevitable?

Yes, sometimes I worry if there is anything bad will happen. But I tell myself, even if I keep worry I can't do anything to avoid it to happen if it does. Thus, I think positively always using the rule of living in the moment. If you keep on having negative thoughts and feelings, I suggest you to read up some motivational books e.g. Chicken Soup for the Soul: Think Positive to count your blessings and having a positive attitude in life.