I hit a psychological wall on my long run

For context: I friend of mine convinced me to sign up for a marathon with her in october which I drunkenly agreed to (I wanted to run one for years anyways). Had to prepare for exams end of july so no real running there. I was only able to really start to train longer runs since august since i had back problems that would show up after 3km; luckily these are completly fixed now.

today I aimed for 25km, never ran that far before. My last long runs where 17km and 20km.
Fueling wasnt an issue i think - knees, back and muscles all felt fine but at 21km I just didnt want to continue anylonger (I only manage a pace of 7min). So after draging myself for another 1,5km my brain said screw that, 2hrs 30min is enough and I just stopped realizing in that moment starting again will not be happening. I jugged down my remaing water and walked the rest home.

Do you guys have any tips to get through such a psychological hole?