Love marriage is allowed in Islam provided the love doesn't cross Islamic boundaries and enter into the haraam. What people find more difficult is determining whether or not they are crossing boundaries.
"The Prophet SAW said it is better for a man to have a steel nail driven through his head then to touch the palm of a strange woman. hadith"
It is true that even hand shaking or hand holding between non - mahrams of opposite gender is not allowed. So if you are holding her hands then yes you are crossing limits and it is haraam.
There are also levels of zina - which are haraam.
Look to try and make her your wife rather than your secret girlfriend.
