Sometimes an easy synastry translates into a challenging composite, and vice versa.
In your case, the synastry has some very beautiful contacts with the personal planets, but some quite difficult ones. And these are reflected in the composite.
You also have contacts with the nodes, descendent, that are very affectionate, very attractive and compelling.
The outer planets are in stressful contacts, most especially the mutual Pluto squares.
Each of you interestingly enough has a close sun/mercury conjunction, which makes you both very much integrated between your ego and your thoughts, you know what you are about. So much so that you don't necessarily need input from others, because you are really focused and integrated. You don't listen much to others, because you intuitively "know".
Perhaps the purpose of the challenging Pluto contacts is to ruffle your feathers a bit, forcing you to look at things from a different perspective and have to defend your position.
One of the sun/mercury conjunctions is in airy intellectual Aquarius, the philosopher, detached and the one who looks at things in a universal point of view, sometimes seeing the forest but not the trees. The other is in emotional, sensitive, feeling cancer, extremely intuitive, caring for others, nurturing, but quick to withdraw within its shell when stepped on by others. So these two have difficulty working on the same plane. They are coming from such different individual worlds.
The very loving aspects are seen in the composite with the Venus/mercury/sun conjunction, while the difficult ones are in the Neptune/uranus/mars conjunction, and in square these create quite a bit of tension in the relationship, which is what the composite represents.
In the synastry his north node on your stellium of sun/mercury/neptune/uranus bring in the theme of "destined" contacts, something karmic between you two, and it is irresistible.
Keep in mind a karmic contact does not necessarily last a lifetime, it lasts as long as is necessary to work out past situations that were left hanging unresolved. It could last, but not necessarily, and it could be romantic or not.
Jupiter in the composite making a trine to the mars/neptune/uranus and in the degree of the sun will be very helpful to keep things together, it is forgiving and generous.
When you mention problems in the past, keep in mind that these are reflected in both the synastry and the composite, so they were not occasional and done with: they are part of the theme of the relationship, so prepare yourself.
With strong self will and determination, any differences between two people can usually be overcome, just be ready to accept that it is a little hard going with this one.