Maintain and nurture your friendships outside the mutual friends that you may share with your SO.

Your own friends can be your sounding board on issues in your relationship and offer advice that benefits YOU however, with mutual friends you never know what will get back to your SO and the advice my be tempered to keep the peace and there may be a lot of devil's advocating that can make you feel gaslit and unheard.

If you break up, you need to have your people who have your back and are in your corner... I've been thinking a lot about this because, when my parents separated my mum was left friendless. All her friends were mutual couple friends who sided with my more powerful dad. She eventually made new friends at her workplace but I could tell it was not just the breakup but the loss of her best friends that really hurt her.

So don't make the grand mistake of only having mutual friends in your circle. If that's the case, start making friends that are just yours.