I'm going to be honest about my opinion on my girlfriend's art
Why do you feel you need to be honest about your opinion of her art?
She needs someone to be honest about her shortcomings
It's great to be supportive of your partner, but just ensure you approach the situation tactfully. She may not be receptive to criticism, but it's good to be honest if you feel it'll help her learn and grow. Has she ever asked for your opinion on her art in the past?
No I haven't done it before seriously.
Well I hope you can be honest and still consider her feelings. Have you practiced what you might say? You can try on me and we can find something that doesn't sound too harsh if you like.