I think you might be confusing subspace (a chemical reaction to play within the brain, creating a euphoria) to sub mind space or head space, (how the sub is feeling and where their thoughts lay in regards to submission ) I know I'm guilty of often miss using the terms but often in "context" they fit so its easy to think they are the same.
To be honest if your not feeling "it", its for a reason! I'd be inclined to look into why.
Are you tired? stressed? feeling disjointed from your partner? Attempting actions or tasks that aren't really within your limits?
What has changed from when you did feel like it? Less personal time to recharge? less time for things that stimulate you? tired of the same ole, same ole and wish it was different?
Feeling like undertaking BDSM is just like the desire to want to have sex or please your partner. Libido is multi faceted. Often its not a one size fits all.
What gets us all in the "mood" is often very different. (*side note does the Dom/mes and subs libido match, this could be a whole other issue. Lopsided)
If you are more asking how to do you get ...hmmm ..into a deeper mind set of feeling submissive. If you owned there several ways a Dom/me could bring that feeling "more" forward...it is really is as simple as him/her making themselves more in the front of your mind through their interactions with you. I use things like reminders that the submissive can wear or see that remind him/her of me or small tasks during the day when we are apart. Other things like positions and mantras will also help you to center yourself in your submission.
BUT...you also need to be careful that your not leaning into relying on your Dominant to micromanage you to feel more submissive. A relationship is a power exchange....if your taking more than your giving and it is requiring more and more work from the Dominant, then maybe there is a greater issue to be addressed and maybe some things need to be re negotiated. Conversations normally solve most things
if your trying to do this solo, unowned. The above would still work but I'd still suggest looking inward to what has altered or changed.
there also the standard classic..more porn!
I do hope it improves for you