My girlfriend keeps flirting with guys. A few weeks ago I came back from the loo, seen her kissing a guy. Last week she showed her tits to my mate. She says she loves me, and said it the drink that makes her act stupid.

So leave her and find one that does not drink, I assume you don't drink ?

If you cannot trust your girlfriend for the 30 seconds you leave the room to pee, what on earth would she do with, say, three minutes.

Any relationship, without trust, is meaningless.

Dump her, or stop her from drinking. Or both.