Smoked weed 4 months ago for my first time. still feeling unmotivated, tired, anxiety, maybe a slight derealization.. when will this end ?

Mental Health : It is unusual for someone to blame their current mental health on a drug they used months previously. This surely cannot be from using marijuana more than 120 days ago. I would advise you to get to your family physician to begin to figure out what is really going on with your mental health.

More to it: It is highly unlikely that you would still have such effects 4 months after one use. If you are still using, then it is possible it is from ongoing use. If not, then you are likely having other psychological issues, such as perhaps guilt for doing it, or you have other things going on, such as obsessive thinking about it, or about something else, anxiety, etc.