I have begun to use the taphsic method, but I have a problem.
Initially, I set the date to one week. I do this because I haven't been for a straight week in what seems like ages. I'll be clean for six days at most and then the seventh day I almost inevitably fall. I know it's weird, but it's true. The problem is that
A) I always fall in the middle even with the taphsic method
After that week of taphsic method, I just can't get myself to renew the Neder because I feel I will fall anyways. Then, I just fall the next day.

Please help me move on!

It seems like counting is just bad for you.
You have a lot of meaning attached to a week and that isn't helping you.

Just forget even what day you started and focus on forgetting about the whole battle and when the lust hits just take it one moment at a time until it ends.

Possibly the taphsic is the problem and it isn't strong enough but it sounds like you might've fallen beause you knew what day it was a.k.a. "sixth day, the one I ALWAYS fall in".

Try to forget the battle, make gedarim that you'll keep in order to avoid a large chunk of the battle and maybe make a knas for a little encouragement.

Try it out and see if it works for you.If it doesn't and I'm wrong then it won't be the first time (third time I think) so keep us updated about how it's going.

Hatzlacha!