Unfortunately, I have to say that with your example the past sounds OK to me.
I think it might be better to say think in terms of preferences rather than only accepting one form, so the differences are more of frequency- one variant may use the present perfect more than another with a certain adverb. Some grammar books will say that in BrE we use the present perfect with recently. We do, but we also use the past. However, in the negative, we almost exclusively use the present perfect. In BrE, already with the past is not at all common and many sentences that might sounds OK elsewhere will sound odd.
In the case of the last three months, the present perfect is the natural choice as the time continues to now. But the past simple does not sound like an error to me- the person could be thinking of being ignored on three specific occasions in that period of time, for instance. If the emphasis is on those occasions more than the time, then a speaker might choose the past. However, other BrE speakers may disagree- we'll see what they say.