I'm going to lie to my girlfriend
Why would you lie to her?
I'm planning to do it, but I'm sure I won't do it
It's wrong to be dishonest with your gf. She'd be really upset with you.
Just trying to say that it might not happen. Just trying to plan ahead.
It would be better to say that it won't happen than to say it might. Lying to your partner will cause problems in your relationship.