OK I have been riding a tri-coloured mare for 3 weeks running and in that time my riding has gone down hill and I'm scared of riding her. What do I do HELP!!

You already know my opinion. You must form a partnership with the horse. Horses may act scary, but inside, they're innocent and worried what you may think of them.

If it was me, I'd take her out barebacked with a hackamore (chain chinstrap) and explore the world together. We'd go for a while, then stop, for her to graze while you lie on your back and make horse noises to get her attention. Just the two of you, hanging out. A mare may be moodier if she's in heat. Tasty weeds can go a long way towards softening her up.

She's not mine and I would never dream of going bareback on her she can be a very moody mare who always wants her own way for some reason I perfer geldings to mares

OK, stupid question, but do you really have to ride this mare? There's no magic rule that says you have to be comfortable riding every horse. Also, no matter how long you've been riding, that doesn't mean you're going to be secure enough in your position, etc that you're going to partner with every horse the way you should. It appears to me, more and more, that you're just getting more and more nervous and are losing you nerve. Maybe the best bet would be to take a step back (or actually it would be a step forward), and go back to riding horses you feel more comfortable on and re-establish your foundation and get yourself to the point where you can get more out of them before you tackle the more difficult cases. It may even be time for a new riding school or new type of instructor. The ideas of "that which doesn't kill you makes you stronger", and "when the going gets tough, the tough push through it" have to be applied judiciously and not to the point where you just get yourself freaked out or hurt. Talk to your instructors. A good one will understand and truly try to help you and not overface you so. Also, don't think of returning to the basics for awhile as any sort of failure, it's intelligence and strengthening your foundation and make you a much better rider in the end (even great musicians have to go back and work on scales, etc at times)
Hope this helps, although it may not be the answer you wanted.

Geldings are generally mellower because they aren't dealing with sex hormones.

Yeah it helped but I haven't got a choice of who I ride really and just a month ago I was cantering now I'm even scared of just trotting she started to nap the little ponies and I can't control her.

Tell someone who can make changes that this mare isn't helping your riding improve. Maybe you'll be ready for her next year.

With musical instruments, the best, most expensive one your parents can afford is best.

With horses, it's the one you feel comfortable with, even if it's not considered beautiful; or you can just get too scared to learn! If a bad relationship with this headstrong mare is detrimental to your lessons, what is being paid for?

You deserve to learn to ride on a horse who is good to you. Once you have more confidence in your abilities, then, a more challenging horse is something that you will want, and ask for.

Thanks for the advice yeah I told my instructor and she's going to have a word with the person incharge of organising the horses so yeah :P