Your sentence is perfect.
I agree with Aisling and Paul that 'weren't' is 100% correct and that you should ignore the people who tell you to say 'wasn't'. 'If it weren't' is a subjunctive to indicate a hypothetical situation.
Just one thing to add:
Previous answers have given the impression that this is a GB/US difference. It isn't. It's a difference between casual informal English where 'wasn't' (the indicative form) is acceptable, and more formal/traditional English where the subjunctive is required. This distinction is common to both GB and US English, and, in fact, educated US English speakers are MORE likely to use correct subjunctives than their British counterparts.