Every time I want to buy something, I resist and save the money instead. I've managed so save/put away a lot of money like that.
「つもり貯金」という便利な表現は英語に翻訳しづらいので、表現の意味をはっきり説明する必要があります。
例えば、
Save the money I would have spent on something else instead.
(物を買わないでそのお金をかわりに貯金する)
Every time I want to buy something, I resist and save the money instead.
(何かを買いたいたびに、我慢してそのお金を貯金する。)
「つもり貯金をしてたら、結構貯まったよ」= Every time I want to buy something, I resist and save the money instead. I've managed so save/put away a lot of money like that.