My therapist told me that this is normal and that you can't rely on your youthful energy to drive your motivation forever. You have to find another source of power from within.
telling your brain "i know you don't wanna do the thing and it'll be very uncomfortable for you at first but you can do it buddy. it'll be fine" is actually surprisingly effective. it sounds too simple but that fuckass organ WILL listen to you if you treat it with kindness lol