Write it now
I’ve checked this many times. If something comes to mind, write it down right now. Then it will be a different you with different thoughts and energy. And to force yourself means to no longer be real.
This is how Pyotr Rukavina maintains his blog:
I generally write in real time: an idea occurs to me, and the idea takes on a life of its own, gets “called to be written about,” and I try to carve out the time to do so right away. I don’t find the process of writing onerous; generally words flow out of me, and while I will go back and edit the words, finessing the meaning, correcting errors, what emerges is generally fairly close to what I wrote down in the first place.
I write a lot of posts via email, a capability that has allowed me to take my writing out of “sitting in front of a laptop” and, really, anywhere I’m struck. Writing on a tiny iPhone SE isn’t the best and most natural environment (though voice-to-text helps), but the benefit of being able to “strike while the iron is hot” outweighs the fussiness of the tool. These days perhaps half of what I write is on the phone.