The first you would use for a situation that happened in the past, that you’re not part of anymore. You could use this when you’re talking about a past job: I didn’t have a break for a long time (when I worked that job). The second you would use in a situation that you’re still in. So here you could talk about a current job, for example: I haven’t had a break in a long time working this job. So both are correct, it just depends on the context!

The first sentence, "I didn't have a break for a long time," is in the past simple tense. It suggests that there was a specific point in the past when you did not have a break for a long time, but it doesn't necessarily provide information about the current situation. The second sentence, "I haven't had a break for a long time," is in the present perfect tense. This sentence indicates that the lack of a break has continued from the past up until the present moment. It emphasizes the ongoing nature of not having a break. In summary, both are grammatically correct, but the choice depends on whether you want to focus on a specific point in the past (past simple) or highlight the ongoing impact from the past up to the present (present perfect).

The others have answered your question. In addition to "have a break", consider the more common expression "take a break": "I have been unable to take a break for a long time" "I couldn't take a break for a long time" Also you can use plural, especially in the past tense: "I wasn't able to take (any) breaks for a long time"

If you still haven't had a break yet, use the 2nd. If you didn't have one for a long time but then finally did, use the first.

I believe Adrienn explained it perfectly. Я с ней согласен. Понимаю, что разница может и не всегда ясно. Но с практикой поймешь. На русском на самом деле тоже есть такая разница, а чтобы всё это понять нужно еще другое объяснение. Если могу как-то помочь, просто скажи, пожалуйста. Буду рад. Всего тебе хорошего :)