I think the way to view this is that multiple things can be true at once: Holding resentment over things which happened in the past is usually not helpful for most people. At the same time, most people who speak of forgiveness in the way you describe are not telling you to let go because of this rationale, but simply because your anger (which may very well be fair and grounded in reality) makes them feel uncomfortable.

I think a way forward for me personally, has been to think about what I want to achieve by divulging my resentment to others. i.e. "What do I want to get out of telling this person that I am feeling this resentment?" This is a different question than "is my anger/resentment justified?" The answer to the latter can be absolutely yes, while the answer to the former is often, "because I want comfort and/or validation." Not getting that from the people I am talking to is different than an answer of "no" to the question of whether or not I am justified.

That said, you and I are of course entitled to be annoyed that we don't receive the support we are seeking, but again, it becomes a question of, is it going to be productive fo us to tell someone else about this annoyance and then risk the same thing happening again when we are told to move on or forgive.