Had better + bare infinitive means you should do something because, if you don't, the consequences will probably be rather bad for you (as in the case of your girlfriend possibly leaving you). The expression (often contracted to 'd better) is grammatically incorrect without had (or 'd), although many people think that you better, for example, is correct; they just miss the (hardly audible) 'd.
And I think Isra meant I had better leave, not live, right?
Useful when it comes to giving advice -you are not comparing something to something else, so forget about the 'better'. Always the same form and followed by bare infinitive. In spite of 'had' you are not talking about the past or referring to a past situation, the meaning is present (or near future).I'd better put an end to this. (by the way, do not let 'd rather' lead you astray, this is preference).
In meaning, had better is close to ''should/ ought to'' but ''had better'' is usually stronger.Often had better implies a warning or a threat of possible bad consequences.
You had better not do it = If you do it, there will be a bad result.
i think,HAD BETTER is used for those actions, which are good to take in any particullar situations
we had better read these structions before we start cooking.
it would be good, if we read the structions first.