Internal conflict is conflict with the characters self/in his own mind. Making a hard decision may be an internal conflict.
Another conflict can be external conflict. That is also hard because many characters cannot control something else. External conflict is conflict that is around a character. Something other than the character. An example is a conflict with the weather, (hurricanes, tornado's).
In my opinion, I think external conflict is a main struggle because that character cannot stop that thing easily. In an internal conflict, many control their thoughts/feelings.
(This is my answer from the other question I answered. But I changed it a bit )