My girlfriend wants to try MDMA and I don't want to be there when she does.
Why don't you want to be there when she does drugs?
I don't want to see her struggle.
It can be painful and upsetting to see a partner struggle with drugs. But, being in a relationship means being there for the tough times too. Talk over the issue and explain why doing mdma isn't a smart decision.
I rather hurt her feelings.
That's really mean. If you don't want to be with a drug user than break up with her.