CS-I had the same problem about 5k out from the end for the first six marathons I did. I had to stop ,my head kept saying 'stop you don't need to do this' etc and that's exactly what I did. ...only for half a minute but that meant missing GFA on one occasion by seconds. The last two marathons were sorted by me deliberately taking a gel with caffeine just before that stage and as a result both led to strong finishes for me and getting GFA on both occasions and a pb. I just think the gel gave me the mental lift to tough out the last few miles and finish stronger than previously. It may have been a placebo effect but it worked.