You have never entered anything without assessing it first
Count the relationships you have entered without first working out whether they would hold, and the number is close to none. Saturn in Capricorn in the 7th house turns partnership into a structural question: not whether this is enjoyable, but whether it would stand.
The 7th house is one other person with an equal claim; Capricorn builds things meant to last and does not commit to anything that will not. So your attachments are few, thoroughly considered and extremely durable, and the assessment period has ended several promising things before they had a chance to demonstrate anything.
The weight is running an evaluation on somebody you like
Being fond of a person and simultaneously assessing their suitability is uncomfortable and continuous, and you have never quite been able to stop doing the second. The other party can usually sense that something is being weighed without being able to name it, which does not make the early stages any easier for either of you.
It comes due when the commitment is actually asked for
The test is a moment when the assessment has to conclude: somebody wants an answer, and no further observation is going to make the decision safer. Your instinct is to extend the period. Deciding on what you have, and saying so plainly, is the specific act, and there is no arrangement of the evidence that makes it feel sufficient.
They think you are unromantic, which is not it
Because you talk about a relationship in terms of whether it works rather than how it feels, partners can conclude that the feeling is absent. It is not; it simply arrives in a vocabulary they were not expecting. Saying the plain emotional sentence once, without the structural framing attached, corrects a misunderstanding that has otherwise run for years.
Something still standing in forty years
What this builds is genuinely rare: a partnership that has been through several eras of both people and is still functioning, because it was constructed rather than fallen into. Most attachments do not survive that much change. Yours does, and the thing that makes it survive is the same caution that made the beginning so slow.
Duty holds the structure and does not make anybody happy
You will honour a commitment through periods when nothing about it is rewarding, and that reliability is the reason the relationship exists at all. It is also not the same as warmth, and inside a long arrangement the two can quietly separate without either party noticing. Warmth here has to be scheduled the way everything else in your life is, which sounds unromantic and works.
Your closest friend has been your closest friend for decades
One-to-one friendships in your life run on a timescale most people do not have: the same handful of names, maintained through several complete changes of circumstance. It is a considerable achievement and it means the list has no mechanism for adding anybody. The last addition was probably a long time ago and nothing about that was decided.
You chose a partner the way you choose everything
The same faculties you apply to a career or a house were applied here: the long view, the durability question, the assessment of what this looks like in twenty years. That produced a sound decision. What it did not do was leave any room for the possibility that some things are chosen well by not being assessed at all.
The joint arrangement is properly documented
You will have the difficult conversation about money, property and what happens if, and you will have it early, because leaving it undefined is intolerable to you. Partners occasionally find this cold at the time and are almost always grateful for it later. It is one of the few areas where your temperament produces an unambiguous advantage.
You became responsible for their relatives too
Attaching to somebody meant taking on their family's practical difficulties as well, and you did it without discussion because that is what you understand commitment to include. The obligations accumulated quickly and none of them was ever formally accepted. Whether they are all still yours is a question worth putting to your partner rather than continuing to answer alone.