I’ve done 18/6 for awhile not but it was extremely challenging in many ways. Your body has to adjust with going longer times without food and especially carbs. The biggest tip I can give you for a 1200 calorie diet is to trust the signs your body is giving you. If you feel weaker somedays than others then trust your body and eat a tad bit more during your window. I do 18/6 seven days a week but I allow myself to have more calories on days I am feeling more sluggish.
There is very little science to support the existence of a fixed ‘calorie requirement’. Also, eating when not hungry can contribute to gastric stress/inflammation/IBS/etc.
On the flip side, energy conservation mode is triggered, when meal size goes down. (Part of why IF is so great: by keeping meal size same as before, the body will continue to think food is abundant and not reduce ‘calories out’ as much as on traditional dieting.)
As long as we supplement with micronutrients (electrolytes, etc) it’s not an issue. Our bodies are consuming the caloric deficit from stored fat.
As far as mental health goes, again an emphasis on micronutrients but equally important are activity and rest. Throughout my journey I’ve learned how certain foods and supplements affect my mood and mind-state, but it’s just as crucial to move around and make sleep a priority. Finally, eliminate all sources of unnecessary negativity and stress; value yourself and others who appreciate you back.