Middlesex started strongly when opener Artemis Downer, who hit an unbeaten 60 when they hammered Glamorgan by 10 wickets earlier in September, impressed with a couple of boundaries before being caught at point off Anna-Mae Shearn.
Izzy Routledge and Sproul helped steady the ship with a fluent 60-run partnership, before Routledge was caught at mid-off from a Nicole Reid delivery.
Sproul soon followed for a solid 43 from 67 balls when caught and bowled by Poppy Walker.
Poor Glamorgan fielding, with several dropped catches and a missed stumping, boosted Middlesex as Horley and Trussler put on 131 before the latter was stumped by Abbey Freeborn off the bowling of Eve Jackson.
Horley’s innings was ended on 117 but she helped Middlesex set a tough target of 279.
They got their breakthrough in the fifth over when influential captain Lauren Parfitt was bowled by Siya Nanavati for eight.
Glamorgan’s woes continued when the hard-hitting Daisy Jeanes was caught at mid-wicket off Issy Routledge.
The wickets continued to tumble when Horley followed up her century by taking the wickets of Bethan Ellis and Megan Sturge as Glamorgan slumped to 72-4.
A 72-run partnership between Freeborn and Gammon offered some hope, until the former was bowled by Anaya Patel for 43.
Glamorgan won the League Two T20 Blast but Hannah Davis extinguished any hopes of a double with two swift wickets, including the key one of Gammon when she was caught in the deep for 58.
It was fitting that Horley had the final say when she had Gemma Porter caught by Trussler.