It's tough getting knocked back like that, especially from something you really want to do.
Have you tried writing to them to ask what you could do in the interim before applying again? Perhaps it's the lack of essay writing experience? Perhaps a lack of maths/science background? Best to try and find out so that you can focus your efforts in the right area.
There might also be some "bridging courses" you can do to help give you some of the university and formal study experience.
Don't be disheartened though, you
can get into some post graduate degrees with no (or part) undergrad. Just a matter of showing you have the capacity to take on formal academic study.
As an alternative, you might want to check out
Open Unviersities.
Good luck and don't give up!
Cheers,
Carl