This is a very frequent problem that happens to people that are in longer relationships, that haven't yet fully developed into the next stage. If you are out with the love of your life, and yet you are feeling lonely, that can be a very devastating feeling. The reason why this is happening is that you are not feeling deep enough emotionally at the moment, and you don't have a lot of conversation topics to chat about with your partner.
Luckily, this can be solved without breaking up, but it does take effort, and a little bit of growing up. For one, you need to know that all relationships may feel less energizing as time goes by, and that is a normal part of life - that happens because you already know most things about your partner. The other part is to try and have conversations with more impact in order to feel more positive emotions.