I am really struggling with my relationship, and I don't know what to do.

I have been with my partner for 3 years now, and they are my best friend. They are someone that I have a great time talking to and having fun with, but there is one thing that I cannot get myself to do. I cannot find myself to be sexually passionate about my partner. For context, I am a bisexual man and have dated men before this. My partner is the first woman that I have been in a relationship with and have been intimate with. I only had one night stands and flings with guys, but I have always planned on being in a relationship with a woman.

In our relationship, I find that I can have sex with my partner, but it was never something that I felt "lustful" towards if that makes sense. My partner wants me to be passionate and lust towards them, but it is not something that I can do. I love them and they mean so much to me, but I cannot get myself to confidently initiate sex with them. I know that it hurts their feelings, and I feel really guilty about it. I have this creeping anxiety about sex all of the time, and it almost feels like to something I have to plan and prepare for. I even struggle to say words like calling them "sexy" or "hot" during sex or outside of sex because it just feels in-genuine. I also should be honest and say that sometimes I think about being with men again but I think that it may be because that is what I know best. I stress all of the time that I am not as attracted to women as I had thought previously, and I don't want to hurt my partner. This news would be devastating.

How do I fix this?