Finally, all of you, have unity of mind, sympathy, brotherly love, a tender heart, and a humble mind.
1 Peter 3:8
There are countless reasons why you would need to mend a broken relationship. The first thing to do is to name them. Is it money? Not spending enough time together? Responsibilities getting in the way? Whatever the problems are they can be overcome.
If things have not totally gotten out of control (and even if they have) you can attempt to fix them by opening the lines of communication. Too often, when things start to go bad, people in a relationship shut down and withdraw into themselves. What you should do is grab your partner and say enough is enough, let's have this out right now. We need to work together to resolve these issues.
If the problem is money, try to either find ways to make more or find ways to spend less. One or both of you go get a part-time job to bring in more cash or learn how to spend less by cutting coupons or buying the store brands which are usually as good as the name brands and cost a lot less. Turn a hobby into a money-making machine.
If you don't spend enough quality time together then start having a date night once a week or once a month. Put a babysitter on retainer and use them frequently. Go see a movie and have dinner, go see a play, have a picnic in the park, or just go for a walk after dinner. Do something to keep in physical contact with each other during your "date". Holding hands will help mend a broken relationship.
I once knew a couple who were married for 73 years, had thirteen children and countless grand and great-grand children. They were so cute together, and they held hands everywhere they went. Physical contact is very important in keeping a relationship healthy.
How about those responsibilities? If they are too much for one of you to handle then ask the other for help. As a couple sometimes one of you just expects the other to know what you need or are thinking. If you think about it that is rather foolish, right? I know I can't read anybody's mind, can you? So lower your expectations and ask for help. Explain things and show them how to do what you need done if they don't know how. Work together to divide responsibilities evenly or if money is not a problem, hire someone to do whatever it is that you need done.
Make some time and go have some fun together. Go fly a kite, go bowling, go to the go-cart track, play miniature golf, find a way to laugh together. Be creative. Play, laugh and be happy - together. Remember how it was when you were all brand new and just falling in love? You spent every single moment together and everything was fresh and fun. You laughed all the time. Find your way back there and you'll also find that's the way to mend a broken relationship.