I would say Saturn is the key idea representing loss thru transits.
Saturn takes about 28-29 years to come full circle. We can expect Saturn to spend some time in each of our 12 houses. Unless you work with an equal house system, however, the length of Saturn's transit through a house will depend on how big each house is. Saturn will spend approximately 2-1/2 years in an "average"-size house of 30 degrees.
Note that Saturn's effect is to crystallize, to ground, and to solidify. Generally, the effect of Saturn transiting a house is felt in a more pronounced way in the first part of its transit through the house, as well as when it forms aspects to natal planets. Toward the end of the house, the native tends to have gained increased understanding—experience, if you will—with the effects. Saturn's movement by transit through the houses acts to aid us to learn to depend on ourselves in different areas of life. At first, we generally encounter feelings of being thwarted or unsupported by the outside world in the particular areas of life that are ruled by the house. By the end of the house transit, we will have learned to look within ourselves for support.
In general, as Saturn passes through the fire houses (the first, fifth, and ninth), we deal with self-confidence issues and how these issues have affected our lives and our productivity. Through the earth houses (the second, sixth, and tenth), we confront issues of self-worth and effectiveness. Through the air houses (the third, seventh, and eleventh), we evaluate our relationships with others and how much (or how little) we can depend on these, as well as our mental outlook. Through the water houses (the fourth, eighth, and twelfth), we re-evaluate our inner stores of faith, our connection with the past, and our psychological workings.
Saturn makes us accountable for ourselves wherever it touches. Each house governs specific areas of our lives, and of our personalities, and transits of Saturn will bring these areas up for "inspection", re-organization, and restructure.
Working "outside in" is best because, this way, one gets a broader overview of the longer-lasting transits and conditions. This provides better context for the refinements that the inner planet transits offer us.
Outer planet transits affect us in large chunks—they can be active before the transit is exact by aspect, but we have seen the following: Many times an outer planet transits our natal point once by direct motion, moves forward and then retrogrades back over that point, only to turn direct again and pass that same natal point for the final time in that cycle. Generally, the seeds of the transit are being sown in the first part of this cycle (when the transiting planet first "hits" our natal point), and the "problem" or crisis reaches a head during the middle part of this cycle (when the transiting planet retrogrades back over the point). Finally, the situation is resolved around the time the transiting planet moves over our natal point for the final time in the cycle. Note: we often "see" or "feel" some sense of imminent resolution when the transiting planet goes direct after its retrograde period, before it passes again over our transited point. If our experience of the transit has been a crisis, then we feel marked release at this point. This is likely because a planet going direct after a period of retrogradation is symbolic, in general, of resolve.