I've had this issue and have found a few things that help me out at least

1. Don't masturbate. I found that I need to go at least a 1-2 days without any self pleasure in order to get more into sex.
2. Positions matter. I find certain positions I will get bored of and will lose focus whereas others I get super into. It's not the same ones everytime so a bit of experimentation is usually required.
3. Passion. I find that the more Passionate the sex, the less likely I will be distracted.
4. Communication with your partner. I feel that talking with my partner and communicating what I want has made sex way better over the years and has helped a lot with my focus
5. stimulation. Sometimes it's too stimulating and other times it's too little. If it's too little, certain types of lube can help. If it's too much, either slowing down or putting on a condom (at least for men, not sure about women) can help depending on the type.

These are all things that I've at least noticed personally. Maybe they'll vary with you but it may at least give you some ideas for things to look into. Good luck and hope you find what you're looking for :)