Outside the slams yeah, he was better in 2013 and 2014.
But at the slams, he was just much better in 2018/2019, at least mentally. He was still very consistent in 2013-2014 going deep but he was losing tons of finals/semis.
From AO 2012 to Wimbledon 2014 he won only one slam despite regularly making SFs and Fs.
Nah, don't see the difference. Mentally similar. Difference in winning is only due to much much better competition in 12-14.
Djokovic was only really tested once mentally in Wim 18-AO 19 in slams - vs Nadal in Wim 18. (doesn't include minor ones like delpo at 4 all in 2nd set or Fucsovics in 3rd set)
Djokovic won 3 slams from AO 12 to Wim 14. He was simply outplayed by better opposition in most of the losses or else opposition was good enough to test him mentally unlike in Wim 18 to AO 19.
Only USO 13 final was a strong letdown mentally. But then he had AO 12 SF/F vs Murray/Djokovic, RG 12 4R/QF vs Seppi/Tsonga, AO 13 4R vs Stan, RG 13 SF vs Nadal, USO 13 SF vs Wawa as net positives mentally worth talking about. (yeah, there are other minor ones, but not strong letdowns mentally)
If you want more proof about Wim 18- AO 19, Djokovic was outclutched or outdone mentally in Bo3 multiple times in Wim 18-AO 19: tpas in Canada 18, Khachanov in Paris 18, Zverev in YEC 18, RBA in Doha 19. Aka opposition atleast somewhat capable to push him in Bo3 unlike in Bo5.
And if you want to bring in Djoko only cared more about the slams, Cincy 18, Shanghai 18 and YEC 18 disprove that. He won his first Cincy, was dominant in Shanghai. Djokovic was dominant in YEC 18 till the final. Lost 8 games max in first 4 matches.It took 8 games in the YEC 18 final for Zverev to crack open djoko mentally by being relentless and serving really well.
Yeah, djoko had moments of roughness on Bo3 in Wim 18-AO 19, like Cincy couple of matches, Paris semi vs fed, but cracks also showed clearly as mentioned above.
Djoko of Wim 18 would still lose to fed of Wim 12 not too dissimilarly. Would lose Murray of Wim 13 convincingly if he got past delpo that is.
Djoko of USO 18 would still lose to Murray of USO 12 in a contested one in windy conditions.
Djoko AO19 would lose to wawa of AO 13/AO 14 in contested matches.
Only things you can really complain significantly mentally in AO 12 to RG 14 about is USO 13. Which 18 USO djoko would have lost in the end vs USO 13 Nadal anyways.
Of course there is USO 14 vs nishi which was a significant mental letdown, but that's outside the timeframe you mentioned.