Should I have stopped striving when the matter became difficult after performing Istikhara, or was it correct for me to supplicate and then strive to achieve the matter?

If you seek Allah's guidance with sincerity and entrust your affair to Him, then proceed with one of the two matters, you are not mistaken, because you have acted upon what Allah has chosen and decreed for you. It is not necessarily true that difficulty in the beginning of an matter is an indication of Allah's disfavor. After performing Istikhara, one who seeks guidance acts upon what is facilitated for them. Therefore, there is no error in what you have chosen to do or leave, as long as your choice was not born of prior personal inclination. And know that Allah's choice for you is better than your choice for yourself.