^

Actually, it's extremely unlikely Mierin was still LTT's lover when he rose to a leadership position, given his age at the Sealing. A woman as ambitious as Mierin would long have forced LTT to marry her if they were together for decades or centuries (neither of them really sound as if they're a real old couple, beside. It's rather all very "adolescent". It's even that appearance Lanfear used to approach Rand at first, the way she looked in her youth, when she dated LTT)

The way LTT spoke of the relationship, it began and ended in their youth. Lanfear latched on the strongest man of his generation, and eventually he realized she was not loving him for himself, but saw only his strength and skills as tools to make herself the greatest AS of the Age (and her career suggests strongly that despite her strength Lanfear didn't have the talent and human skills, and the dedication to work hard to get what she wanted probably, nor the mentality for selfless service that was a way to climb socially in the AOL, to become of the great AS of her Age). So LTT put an end to the relationship.

When LTT said Mierin loved power, he seemed to speak even more of the OP than social/political power. That makes sense, because this remains the obsession of Lanfear's life. The strongest man of the Age, her obsession with this new source of power, her obsession in the series with the CK and san'angreal etc. Lanfear was a middling scientist among geniuses as the most advanced research center. She didn't seem interested one bit in politics. She's not Barid Bel Medar. Her obsession with power is more a god-complex.

Many feel LTT was the bad guy in this, but now that we know his age, there seems to be another story there. LTT was deeply in love with Mierin, and realizing she didn't love him but was using him broke his heart. We can't know for sure, but what happened with Lanfear seems to have driven him away from (at least serious) relationships, until he met Ilyena centuries later. Until Ilyena, he may have feared the Mierins/Berelains too much to let himself fall in love.

That may be what spited Mierin so much: his loneliness pleased her ego, as he had never replaced her, until he finally met Ilyena and married her 50 years before the WOS.