I'm sorry you're feeling like this. It's not a pleasant situation to be in, and it's completely understandable that you're feeling left out and maybe even a bit insecure. Here are a few steps you can take to address this issue:

1. **Communicate Openly**: The first step is always to have an open and honest conversation. Find a calm moment and let your partner know how you’re feeling. Use "I" statements to express your feelings, like "I feel neglected when you spend more time with your friend than with me."

2. **Express Your Needs**: Be clear and specific about what you need from your relationship. Maybe you need more quality time together or certain activities that are just for the two of you. Let your partner know how they can help make you feel more valued and secure.

3. **Listen to Their Side**: Relationships are a two-way street. Give your partner a chance to explain their perspective. Maybe they haven't realized how their actions are affecting you, or perhaps there's something else going on that you’re not aware of.

4. **Set Boundaries**: It might be helpful to establish some boundaries that are acceptable for both of you. This doesn't mean you should control who your partner can or can't hang out with, but finding a balance between friend time and partner time is important.

5. **Plan Together Time**: Make an effort to plan activities together, whether it's a date night, a weekend getaway, or even just a quiet evening at home. Creating special moments can help strengthen your bond and make you feel more connected.

6. **Evaluate the Friendship**: It's also important to reflect on the nature of your partner's friendship with this girl. Is it purely platonic and does it pose any threat to your relationship? Sometimes jealousy can cloud our judgment, so try to assess the situation as objectively as you can.

7. **Seek Support**: If you find it difficult to navigate this issue on your own, consider seeking the help of a relationship counselor. They can provide a neutral ground to discuss your concerns and work on solutions together.

Remember, it’s important for both partners to feel valued and secure in the relationship. Addressing this issue with love and openness can help you both find a better balance. Best of luck!