I'm here!
Right, OP in an ideal world you would do a one-off visit to a hairdresser to sort your roots and give you a t-section of highlights to lighten the overall effect, if you are feeling "blocky" and dark, otherwise known as "a bit Pauline Prescott "
The reason your hair is fading is simply because box dyes use a fairly pissweak form of peroxide suitable for home use by novices. What you really need to open up grey hair is 20 volume developer (peroxide) which is 6%
A good mix for you to maintain your roots, particularly if you have a few highlights to break it up but want to keep your base colour a mid fairly neutral brown, is 1 part 4.3 Majirel with 3 parts 5.0 Majirel which you then mix with 1 and a half times 20 volume developer
So if you measured in half teaspoons, you would have half a teaspoon of 4.3, 3 x 1/2 teaspoons of 5.0, and 3 whole teaspoons of developer. You paint it on and leave it on for 35 minutes.
This would be plenty to cover roots in terms of amount.
Buy a tinting brush or even a foundation brush for very precision application to the roots only and a big pack of latex gloves.
This would allow you to maintain your roots at home with professional grade dye which will not fade ginger at the temples, will completely cover the grey, and give incredible shine. It smells lovely too!
Credit to beloved style and beauty queen otchayaniye for this info - it was her that got me into professional dye (though I use a mix to achieve auburn). Now, it could be that this mix is not right for you but it sounds like it will be - in an ideal ideal world you'd visit a hairdresser first, have them do your roots with whatever they suggest and a t section of highlights, and try and find out what they used (!) - if they bring over a swatch book memorise the numbers!!! And try and use someone who uses Loreal because Majirel is best for covering grey, it is said. But I honestly feel compared to what you have been doing if you buy this combination outlined above and do your roots only, you will be much happier with your colour and then you can go and have highlights if you want to lighten up a bit. You could get away with a few highlights 3 times a year which will not bust the bank.