She seemed to be softer and kinder and less aggressive in TNG, in DS9 she became a tyrant.
Well part of her attitude to begin with on DS9 was that she didn't want to leave the E-D and go to an old station to begin with. Once she got there she didn't have a place and felt useless. She then got her school but had to condent with religious nuts, a dwindling school role, before closing it down. She then got the chance to turn her hand back to her passion and headed to Bajor, then the war broke out and she returned to Earth to be safe.
All of that would apply strain and pressure to any relationship. So she may snipe at Miles every so often, but at the core of their relationship they are very much in love.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)