I signed up to GYE 3 days ago....determined to complete the 90 day challenge. And here I am 3 days later after I just fell into the lowest. 
honestly it feels that my urges get stronger in response to my determination to do teshuva. I understand it helps me if I manage to resist....but G-d, you know me. I'm addicted! How can I resist?

You resisted for almost three days! Don’t let the yetzer hara convince you that you didn’t accomplish. Just brush yourself off and keep climbing.

There are tools to help in this struggle and hopefully you will learn to use them successfully as you continue your journey but the first step is to understand that every effort you put in is an accomplishment of great value even if you fall afterwards.

Abochur wrote on 15 Aug 2022 07:16:

Welcome aboard you've come to right place. keep posting and browse the forums and bezras hashem you will be matzliach.  

90 days can seem pretty intimidating.
I find it helpful to remember that all I need to concern myself with is today.
I just need to get through today.

Ok, I wrote a whole reply and it disappeared, which is generally a good thing. Trying again more intelligibly.

I can only go by my own experiences starting out, there's a point where it suddenly flares up to being unbearable. It's like holding a hot metal railing, once you get past that point, it dies down for a while. You just need to try whatever it takes to get past that point. Restrictions you dont know if you can keep up long term, threats and promises, distractions, whatever. Force yourself to think of other things, avoid things you associate with the taiva.
Other people have more concrete advice, so look around.
Any specific aspect you can identify that caused the trouble this time?

Falls are normal in the beginning, if you signed up for the 90 day challenge you might have received an email stating that and elaborating on that. I would also like to say that kedusha works like a balance, so the stronger pull one has to tumah or however tammei one feels, it should show him the potential of kedusha that one has, because it would be in balance and this would simply be on the opposite end of the spectrum from where one may feel he has stooped to. 

So you definitely do have to koach to resist, just like I do, it's just to what to resist against and I prefer the word fight instead of resist. If you're fighting against using an unfiltered device to watch p, a fall is inevitable speaking from prior experience. In order to really fight this you need to be proactive and do something about the device instead of trying to hold yourself back from using the device, as in go on the offensive. This is just a specific example that may or may not apply to you, but hopefully it gets the general point I'm trying to make across.