How we cite our quotes: Citations follow this format: (Act.Scene.Line). Every time a character talks counts as one line, even if what they say turns into a long monologue.
Quote #1
MARY
It makes is so much harder, living in this atmosphere of constant suspicion, knowing everyone is spying on me, and none of you believe in me, or trust me […] If there was only some place I could go to get away for a day, or even an afternoon, some woman friend I could talk to – not about anything serious, simply laugh and gossip and forget for a while – someone besides the servants – that stupid Cathleen! (1.1.207-209)
We can see here that, even without considering her own addiction or Edmund's illness, Mary's life in the house is intolerable. She has no one to talk to other than Cathleen, and she feels stifled and lonely. Her life consists entirely of waiting for the other Tyrones to come home and dealing with the servants. This boredom surely contributes to her drug habit.
Quote #2
MARY
That's what makes it so hard – for all of us. We can't forget. (1.1.228)
Here, Mary points to the main reason she suffers. We're clued in, for the first time really, that what upsets her most may not be crumbling family ties, Edmund's health, or her lack of social life. Rather, she is haunted by something in her past, including but not limited to: the death of Eugene and the losses of her religious life, innocence, reputation, sense of home, and/or virginity.
Quote #3
MARY
You're welcome to come up and watch me if you're so suspicious.
JAMES
As if that could do any good! You'd only postpone it. And I'm not your jailor. This isn't a prison.
MARY
No. I know you can't help thinking it's a home. (2.2.21-23)
A revealing comment from James here. The sentiment is fair enough, but check out the order of operations – first, he explains that he would come up if he thought it might help, but then he adds that he's not a jailor and doesn't want Mary to feel imprisoned. The message here is conflicted, and it's obvious he would stop her if he thought he could do so permanently. More importantly, as Mary herself points out, he fails to recognize that this house is a prison for Mary. She sits around lonely all day, and even if he isn't actively in charge of her imprisonment, he is the one who originally locked her up.