Captain Colin Ingram was left ruing his side's fielding after James Hildreth was dropped on 63.
The Somerset batsman went on to hit a career-best 159 off just 125 balls as Somerset set Glamorgan 373 runs to win in their Royal London One-Day Cup clash in Taunton.
"We had opportunities in the field but a key moment in the game was putting Hildreth down (when he was) in the sixties and he played really well after that and put us under pressure," said Ingram.
Ingram top-scored for Glamorgan with a typically beligerent 85 off 70 balls but was disappointed he wasn't able to convert his innings into a match-winning effort.
"Batting-wise, a few of us got in and got out, that was disappointing, it was nice to spend time in the middle and I feel my game's ticking over well. But if I can turn that into 150 and some-one else chips in, we'll get a lot closer."