Hey Danielle - cool point you are taking on. I actually am on Day 4 of no Coffee because I could see for myself, just like you, there was a dependency relationship attached to it, and it was time for me to take on the challenge of seeing who I am without it.
A really important point you made here is,
Coffee isn't a bad thing, just with the relationship that I've had with coffee and the affects it has on my body,
this is crucial in realizing whatever we see we are addicted to, or have created a dependency toward - that it's not actually about that 'thing' we are using - whether coffee, food, drugs... it absolutely is the relationship we've created towards it, so awesome you can see this for yourself.
Your starting point seems to be clear on wanting to stop for 21 days, so I say go for it. I can share that the first three days I did exerience some physical symptoms of what I would consider withdrawal, but I in a way also anticipated that because of the amount I drank of coffee, and the fact that yes, it is a stimulus, I prepared myself for perhaps some uncomfortable experience. But what I actually found more prominent than anything else was the emotional attachment I created towards coffee, and that was creating most of the discomfort. I found for me coffee was comfort, and I placed self-comfort in coffee, and so an obvious point of self-separation here... though also cool to see because in that, I see now I can start redefining self-comfort and practicing living comfort of an expression of myself, and not found in something outside of me, such as coffee.
I also thought day 1, oh my - what have I done... why would i do this to myself. Though I realize there are many ways we can manipulate ourselves to stay the same, and not step out of our routines, or comfort zones... so yes, while you may face points of emotional reactions, and discomforts, and even appealing thoughts the justify not doing the 21 days, you can still remain within your decision, and commit to the 21 days. you have the tools. I find also writing about my experience daily in relation to the letting coffee go supports to remain within that direction/decision.
I look forward to reading more about it! Have fun!