I know that んだった and んじゃなかった can be used to say "Should have done" and "Shouldn't have done". I am wondering, however, if they can be used as a past version of んだ Take for example this conversation:
A friend: You never spend time with me
You: 忙しいんだ (The thing is, I'm busy)
Now I try with んだった
A friend: You never spent time with me
You: 忙しいんだった (I was busy, that was why)
Is this incorrect or correct?