Tips and Tricks
1. Realize that there are a MULTITUDE of people out there that are "BETTER" than you. (I know this seems like a weird way to not be insecure, but stick with me!
No matter how smart, rich, strong, funny, recognized in my community and really, really, incredibly good-looking I am, there will be specimens of my species out there that can out do me.
2. Realize that your partner or whomever you are afraid of losing probably knows this as well. That what you offer is unique and either they enjoy you or they don't.
A trick for handling rejection:
I. If someone doesn't want to spend time with/be with me, I don't want to be with them. I only want what is consensual.
I also don't want to be with anyone that I don't want to be with, just because they want to be with me and are giving me attention/gifts/&c. (I see people do this.)
To want to be with someone who isn't interested is, well... to put it harshly, it's pathetic. I'm totally not interested in being pathetic, so I get over it pretty quick. This is something I sort of realized a long time ago and I certainly had situations where I was pathetic, but with practice I learned to get over it faster and faster to the point now if I start to flirt with someone and they don't show an immediate interest I move on with no sense of rejection or hard feelings.
I've even had girls "chase me down" and try to flirt with me after initially giving me the cold shoulder. My theory is they saw my confidence and confidence is one of the single most attractive things ever.
Basically, both of these "tricks" are ways of understanding and building your own confidence using a logical train of thought:
#1. You are the only you and people like/love you.
#2. If someone doesn't want to be with you, it is illogical to want to be around them.
One more to make #3: Appreciate yourself and take opportunities to build your confidence whenever possible. This can go from just admiring how good you are at something to putting the time and energy into yourself to develop yourself (read a book, play some sport, learn a language/take a class, meditate or pursue something you think is cool.)