I wasn't able to follow what you were saying after a certain point.
I was able to follow what you were saying partway, but then I got lost.
ーI wasn't able to follow what you were saying after a certain point.
「ある時点以降、あなたの言っていることについていけなくなった」
to follow what you were saying で「あなたの言っていること・話についていく」
after a certain point で「ある時点以降」
ーI was able to follow what you were saying partway, but then I got lost.
「あなたの話に途中までついていけていたが、その後わからなくなった」
partway で「途中まで」
get lost で「迷う・わからなくなる」
例：
I was able to follow what you were saying partway, but then I got lost. Could you please explain that again?
「あなたの話に途中までついていけていたが、その後わからなくなりました。もう一度説明してくれますか？」
ご参考まで！
カナダ 
