Any girl who says its ok for her boyfriend to do something when she doesn't mean it and has a go at him later is a *****.
She said its ok, so go.
If shes not ok then its her fault for lying.
Not all girls are crazy and not all girls care about valentines day.
You have 364 other days of the year to spend time with her/buy her gifts/celebrate your love for each other.
...and how to stay positive