The real question is: Why don't you believe your girlfriend loves you?
If someone tells you they love you and you're thinking "No you don't!" then it's because YOU feel they aren't doing the things you need in order to (feel loved).
1. Has she proven to be a liar during your relationship?
(If not then you have no reason to doubt her)
2. Is she more "practical" than "romantic"
(If so it just means you are not with "your type of woman")Maybe you need someone who is more affectionate
3. Do YOU Not Love yourself?
(Which means you don't believe anyone else could possibly love you?" or you never got over being betrayed/hurt by an ex-grilfriend)Are you making her pay for the sins of an ex?
If you're going to enter into a new relationship you have start off with a "clean slate". Her honesty and integrity should earn your trust overtime.