Remember that all historians are biased – read multiple perspectives
“History” is a product of human beings. History of the same events and people may be done (will be done) differently from generation to generation. Sometimes, due to advances in archaeology or new discoveries of old texts, history done 500 years after the fact will be better than history done 100 years later. Similarly, changes in dominant ideology might make later history less reliable than earlier historical works. Best to read histories from multiple perspectives and times.
Remember that all humans are complex (like you)
All of the characters of the past were just as you are: receptacles of both good and evil. Remember this when judging them. You should not expect them to be better vis-a-vis the ethics of your society than you are vis-a-vis yours. Better to judge them against universal, timeless virtues (courage, honesty, etc) rather than fickle ideology. A good phrase to remember is one I recently read as paraphrase from Edward Gibbon: “‘their imperfections flowed from the contagion of the times; their virtues were their own.”
The “lessons of history” are not often clear
It’s pretty hard to establish straight cause and effect even in scientific experiments. Don’t expect history to be simple in its lessons. Multiple variables play a role in any outcome. So be careful in drawing out prescriptions from the historical past.
Use the edifying bits
Reading about past persons should inspire you to do great things yourself, or to be a greater person. History at its best connects us to our forebears in a way that highlights their virtues, which are often lofty enough to give us an engaging mission and meaning for life.
Be loyal opposition to past
Your ancestors (and their peers) did things that made you possible. They also did things which would horrify you. To them you have two responsibilities: reverence and critique. Don’t just throw them under the bus. Critique them, yes, but recognize the good they did also.