I dare not attempt a recap of Being and Nothingness, for fear of committing abject butchery of the topic. I freely admit I didn't understand half of what I read. But I did glean some insights from the text, and think it is worth reading, re-reading and studying.
We are free because regardless of any situation we find ourselves in, we have choices. What we choose will be based on what we value most, and what we value most is based on our image of ourselves, in part from choices we've made in the past. We are 'projects' of ourselves. We can make radical breaks in choices, and remake ourselves into new projects. But to try to deceive ourselves about our freedom or our essence is to act in bad faith.
Although we did not 'ask to be born' we are nonetheless fully responsible for our lives; choosing not to choose is still a choice. Our freedom consists in what we do with what has been done to us, including what we have done to ourselves in the past.
There are things in Sartre I do not agree with (for instance, I think he'd be appalled by the idea of a sexual 'orientation' - for him it would always be a 'preference'), but I find his radical assertions of freedom and responsibility to be challenging.
I have other philosophy texts to consider, but I am not through with existentialism. Aside from an anthology of Sartre by Cumming, I also have two anthologies of existentialist texts in general (Kaufman and Marino) and a study of Mulla Ṣadrā that takes an existentialist view of his philosophy. And sooner or later it will be time to read Camus's The Plague for a third time.