HADN'T is used in a past context. In other words, when you're talking about a situation in the past, HADN'T would refer to the time BEFORE that time: "On my tenth birthday, my parents announced that they were getting a divorced. Thing's hadn't been going very well for some time." This means that things weren't going well up to and including the boy's birthday as a point of reference in the PAST. HAVEN'T has the same meaning, but leads up to a point in the PRESENT: "This morning, my parents announced that they're getting a divorce. I'm not surprised - things haven't been going very well for some time." This implies that things have been choppy between them up to and including the present.

It depends a bit on the context. I would need to know more about what you will say next to answer correctly. ♥️

I can teach you english if you want as well as french, spanish and portuguese

The second sentence because your intention is to talk about an action that have place since a past time, if you use "had" you would be talking about a past moment that already had an end.