Man, I've been there.
And if it's any help, the feelings do not last forever.
You're right in one sense: no relationship is guaranteed to last. With that massive reality check on yourself, though, you must realize that if you don't work to become happy with who you are alone, you might just get wrapped up in another relationship and put everything you have into that person. Believe me, I've done it, and I have to constantly work to remember who I am, apart from any relationships... to keep my own momentum and life going so that if I do lose my loved ones (either by breaking up or by them dying), that essentially I will be able to recover myself, heal, and move on to make other relationships.
'Cause it don't do ya no good to believe it's okay to be alone, and to surround yourself with loneliness... even though no relationship truly can last forever (unless we all starting living forever), that doesn't mean they aren't worth the investment while we have the time with that person. I dunno, maybe it's cliche... but it's the whole death-as-a-part-of-life thing that I've been trying to get used to lately. The more we are scared of death, or of losing someone, the less we are able to just let go and
be in the present, for all its joy and pain.
Do not be afraid of being alone... settle into the grief for now, let it take hold and run its course for a while... I know, it just SUCKS. There's no way around it. But throughout, let yourself become who you are without people's influence on you, for a while... learn to value yourself, learn that you are your own best company. THEN, with that solid foundation, go out and show the world how beautiful you are.
Hope this was helpful, some part of it at least...