Perhaps where pride becomes a deadly sin is when it is out of alignment with the actual achievement. Christians teach that pride comes from failing to acknowledge God, or something larger than ourselves, as the creator of that achievement. This idea serves to keep pride in check, and it aims to balance us with a sense of humility. After all, we human beings do have a tendency to embellish. With our pride, we may be prone to expand our sense of achievement beyond what is merited.
Where pride becomes unhealthy is when we distort our sense of accomplishment or value in proportion to the situation. Where we may fall into elaboration is if we give ourselves merit beyond the specific achievement of that moment. For example, maybe we aced a test, and instead of just feeling proud that we knew the information and executed it, we distort that into feeling as if we could ace all tests or are now an expert on the subject matter. The key here is to not get ahead of ourselves.
As I mentioned before, we can also understate, even diminish, our accomplishments, and that, too, can leave us with a distorted sense of self. Healthy pride is therefore having a clear and accurate understanding of our achievements where they are warranted. Unfortunately, there are many experiences in life that can lead us toward having an inaccurate read on ourselves, trauma and abuse being predominant.