Just to be clear this is generalised and not about tenz/kyedae
very dependent upon the individuals and their relationship + life circumstances but speaking from experience the "staying with x through tough times" stuff can be rough.
It can easily build resentment on the part of the helper/stayer because you have to divert a lot of your time/energy to your partner and if your life is already going through a busy/stressful period it really wears on you.
you may also feel like you cant express your struggles to your partner because theyre already suffering - so you bottle up more and feel less comfortable opening up which can also build resentment since you were able to be open before and it changed - but also is just unpleasant in general ofc
and possibly you set expectations for their recovery and, as recovery often is, there are setbacks. then your expectations dont get met and perhaps you wonder "will this ever end" or "how much more"
these are just possibilities and hypotheticals and im trying to generalise just to paint a rough picture. i dont know tenz/kydedae so i dont wanna speak on them