Should i use a martingale?

hi everyone, i've recently taken on a 15 year old mare who certainly lives up to the name moody mare to say the least! she hasn't been worked properly for at least a year maybe more, i've been gradually bringing her back and her fittness has definitely improved however She's very moody and when you make her do something she dosen't want to she will do it but sh will argue for quite while eventually give in but paddy about it on the way. her way of paddying is shaking her head quite a lot and trying to get the reins out of my hands. The last couple of times i've been out with her it has been really difficult controlling her so i'm wondering about getting her a martingale just a standard running one thats not harsh because she is perfect doing everythign else its just if she dosen't want to do something. I was thinking of jsut putting her in it for a couple of months or so untill she finally realises that she is going to have to work for her living if you know what i mean. Just wondering on anyones thoughts on this and what kind of martigale i should try on her if one at all, she ridden in a d-ring snaffle and i've had all her tack checked and it all fits her fine so shes just being a stubborn mare who dosen't want to go back to work..... thanks for any comments :) p.s perfer not to spend much been looking at standard daily martingales and their resnably cheap i'll spend about �20 at the most but one i founf on robinsons was only �11 which would be great :)