It's not fair to set an ultimatum to the ones you love by saying 'I don't like this so you can't do it or I'll leave.' It is fair to say 'I didn't like when this happened, it made me feel (insert emotion here). Can we please talk about how we can avoid this in the future?'
I just want to point out, it's not fair or responsible to say, "When you do X, it makes me feel Y." No one can MAKE a person feel something. I know it's common to say that: "You made me feel jealous, you made me panic," but one's reaction and feelings are one's own.
Better to say, with understanding and by owning your own emotions, "When you do X, I feel Y."
In this case, the OP seems to have issues with panicking in general, which she deals with by abusing drugs. Panic attacks usually stem from childhood trauma. Perhaps the panic/jealousy stems from fear of abandonment.
Many people new to poly freak out thinking about their partner having sex with a new person. Usually they think one or both of 2 things: 1) the new partner is better at sex than I am, or 2) the sex my primary is having with their secondary will make them fall deeply in love and leave me for her or him.
Quite often we think we will be fine with this or that our partner does, when imagining it, but then when it really happens, we feel differently.
Talk it over. Tell your h you are struggling. He and his new gf, if they care about your feelings at all, may agree to slow down the sexual contact until you feel more reassured and secure.