Actually the dilemma you posit and request feedback for is not so much a dilemma as it is an opportunity for you to shine some light into the 'dark' spaces of yourself that don't oft see the light of day.
you state that this one is your favorite yet only part time.
she has been 'broken' yet retains autonomy.
the decision and who has the power to decide is not clear and you fear to 'push' the issue for the risk of losing her.
The darkness is your uncertainty. All of those aspects that we all enjoy in being Dominant are, I am quite certain, present in your relationship with this sub, yet you allow part time submission.
If she is your favorite then you must ask within yourself what course to pursue.
allowing her to leave you for a time will absolutely result in you receiving back a markedly different person from the one whom you release. this is human nature and cannot be changed, expereinces change us and expereinces not shared seperate us.
you need to decide within yourself exactly how you feel about this submissive. My 'gut' instinct from reading your post tells me that you may have the 'Love' for this one rather than the 'love' a Master has for His or Her subs. If this is the ONE then you must tell her and let her go to return and then bond yourself to her to the exclusion of all others. Seek within yourself for this calculation. I offer the following paradigm, think of her as dead and buried gone for all time and examine yourself for the reaction to her absence. this will tell you more about how you feel/need her than any dungeon excercise ever could.
Now that assessment is based on my 'gut' telling me the possibility that you love this sub. If that is a wrong assessment and she is just your favorite peice of fluff at the moment then by all means allow her to depart and step back the relationship to an earlier stage.
Online domination is a wonderful venue for experimental domination. Order her to get a webcam and to link with you as you see fit. control her with set orders, regulate the times she may use the toilet for example, control her feedings, her orgasms, her very core.
I am certain you know how to dominate and I won't list it all but the fact that you even post a query engenders the notion that this is indeed the "one" you cannot bear to lose and not the newest fluff for the grist of your mill.
I offer congratulations for I also have found and kept my "one" and have never regretted nor looked back.
Respectfully submitted