why do certain thoughts stop working?

I’ve been in SCS since January, and it has been awesome. Early on, I found a thought that seemed to change everything. I had been so bitter about a health problem I have and felt resentful everytime I had to take my medication. This led to missing doses.
Then I found the thought, “I get to take my life saving medication.” I just felt so much gratitude, that I was no longer resentful about taking my medication (and still am not). I started taking my medication regularly, and felt grateful every time. For months, I rarely missed a dose. I thought that this thought was so powerful for me that it would work forever to help me take my twice daily meds.
Now, 6 months later, (including a move and changes in routine), I find myself frequently missing doses. I try to use that same thought, but it’s not getting me the result I want (of not missing doses). I know that I could do some models and find some thoughts to get me the results I want (thanks to all my months of work in SCS), but I find myself wishing that I could use the same thought I did before and get the same results that I did before.
My question is, why can I not use a thought that once worked so well for me?
How often do you have to have to change your thoughts to get you the same results?