Kitty
To be honest I don't have a clue why you keep being rejected.
But I would guess that at 19 if you have had some debt and you look eager ( no. of applications) to apply for more credit, from a risk point of view, this is scaring companies off.
My advice for what it is worth, is to sort out a budget for your spending and look to save (say) £200 a month. Put this away over the next 6 months or so and then look to see whether your position has changed. Don't be tempted to go to sub-prime type lenders who charge silly interest rates and then increase these massively if you were ever late with a payment.
At the moment companies are being very risk adverse. There are loads of people in the same situation as yourself, as noted by the number of posts added to this site. So you should not take this too personally.
Huckster