jules, I'm 25, and I'm not in any kind of relationship either and the only serious girlfriend I had cheated on me, and most of my friends, with a few exceptions, are in long term relationships and a few even have kids. It's nothing to worry about. It'll happen for me when it happens, and no amount of worrying is gonna change that.
I really don't get this 'scared of life' thing though. I have no idea wht is gonna happen to me tomorrow, next week or next year, all I can do is make plans for the future and rely on myself and my abilities. It helps to have good friends to rely on, and I do, but at the end of the day, you are responsible for yourself. No-one is gonna do things for you.
Now, maybe you could get run over by a bus tomorrow, maybe you're boyfriend/girlfriend could leave you tomorrow for someone else, maybe anything could happen. The question is, do you really wanna waste you're time dwelling and focusing on the negative? Do you really wanna put a dampener on the good things in you're life by thinking about when they will end or turn bad?
The reality of life is that all things, eventually, end and there is nothing at all we can do to change that. All we can do is enjoy the good times while they last, roll with the punches and learn from the mistakes we make, no matter how painful that is.
Oh and most importantly, believe in yourself.