To reiterate previous answers:
Check the deeds. If they give you a right of access she cannot deny you access. Also, if you have enjoyede unrestricted access for more than 15 years then a right of way exists.
However, if leaving the gate open is a problem then discuss alternatives with her, such as you both having a key or fitting a 'closer'!
As is mentioned on this board frequently, it is much better to discuss problems with neighbours and reach an amicable compromise, rather than reaching for the law straight away!