My guess is it’s harder because you are restarting. I find anytime I stop fasting I have to ease back in if I want it to be manageable. Otherwise jumping in cold turkey is so difficult. I also eat keto which does help - carbs cause so many more cravings for me.
Fasting is more difficult when you reach a healthy-low bodyweight in my experience. After pregnancy I found intermittent fasting and even 36 hour fasts easy. Now I am around my normal bodyweight I can only miss one meal a day maximum, or I get very hangry and irritable, brain fog etc etc.
When I gain after a holiday or prolonged indulgence, I find it very easy to fast again until I reach a certain bodyweight.
Based on my own similar experience trying to restart fasting after almost reaching my goal and wanting to get that last bit, I think you’re right that part of it is physical, part mental. Your body was able to draw on fat stores that aren’t there anymore, so when you decrease food intake now it’s going to fight you on what you’re doing to it, thinking it is in danger and needs to motivate you to get food to survive. Mentally too, you likely don’t have as strong a motivation as before to fight through the discomfort. Oddly enough for me I started doing longer fasts and that change in routine helped me through, but it sounds like you don’t need that.