At first, I thought I could work two jobs, but I couldn’t.
At first, I thought I could handle working two jobs, but it was impossible.
最初に = at first
ダブルワーク= “work two jobs”
ダメだった = I couldn’t, it was impossible,
あとは”it was too difficult”=”難しすぎた”という言い方もできます。
2)の”handle working two jobs”とは “ダブルワークするのを取り組む”とか、取り扱うという意味で、このような時によく言います！
もう一つの例文！
I thought I could handle going to school and working, but it was too difficult.
アメリカ合衆国 
