First of all, there is no question, as the Buddha's disciples realized when they thought about it, that if you truly love someone you will suffer. You will suffer when they suffer because you feel for them and do not want them to suffer. You will suffer when they do something the impacts you negatively because you will think how can someone who loves me do this to me. To name just two examples.
There are several ways, however, to insure that you don't suffer or at least suffer less because of your love. This first is, do not attach to your loved one. That in no way lessens the love you feel, but it means you don't feel co-dependent; you don't feel that if your loved one died, your life would be empty, you would be lost. While it may sound strange to say that you aren't attached to your loved one, if you keep in mind the specific meaning of non-attachment in Buddhist teaching, then you will see there is no contradiction.
Next, an important basis for true love is faith and trust between the two people. Unfortunately, people who love someone have a tendency when that person is facing a problem, is disturbed, of thinking that they can step in and fix the situation, whether through advice or some other means. Such action backfires in two ways.
It says to the person that you don't have faith in them, in their ability to solve their problems by themselves, which in turn impacts their own faith that they can solve the problem. Whether you are dealing with a child or an adult, this is a critical aspect of self-confidence. You do not want to do anything that takes away your loved one's self-confidence. So when these situations arise, you just have to stop yourself and say to yourself that you have faith that all will be ok and the person will work the problem out for himself.
Another reason why we have this tendency is that we see the present situation as a portent of the future. But just as you must come to know that things are the way they are because it's just the way it is, and you will be ok, safe, because it will all work out, you must feel the same regarding your loved one. You should have no desire that his or her life be any different from the way it is right now at this moment.
Suffering is a part of life, unless you're enlightened. It comes with the territory of being human and living in this world. Yes, the further you progress on the path, the fewer occasions of suffering you'll experience. This results from your changed perspective on yourself, life, and other people. But regardless how far someone has come on the path, they will have moments when they suffer.
Regarding the second example of suffering I gave, you must understand that your loved one is only human. Unless he or she is enlightened, he is subject to emotions that are a product of his life experience. Even if the person has advanced far on the path, there are still moments, subjects, which will cause his mind to intervene and produce emotionally-influenced actions over which he has no control. It's just the way it is. You must recognize that your loved one is human and the fact that he has such lapses does not in any way impugn his love for you.
I'm talking here about a relationship which overall is deep, filled with trust and respect. If a relationship is in general fraught with problems, then the balance is different and love is reasonably questioned by such actions.
Bottom line, you can minimize the extent to which you suffer because you love someone, but you cannot totally eliminate it. It comes with the territory and a deep abiding love is well worth the suffering it brings.