I am unsure if that makes you not a longtermist. My understanding of the basic longtermist claim is as follows:
Future individuals matter as much as those in the present
We should do something to positively shape their lives
I think this has little to do with forecasting accuracy, albeit if we were really good at that, it would help us better outline the something in the second point. I think the major part of it is about how we assign moral value to those who will exist in the future.
The Levels of Donation post contains lots of good advice on the same.