How can two individuals live so close together that long?
My wife and I are a couple for 26 years now and last Wednesday we have had porcelain wedding day (20 year anniversary).

I want to share you some hints, how we could get so far, because this is not self-evident:

Have a base

Every partnership needs a base. When I say base, then I mean something fundamental, that both lives stand on. So it is very important to find out, what each partners base is (see also my post "My Deal with God - Preamble").
This could be for example a honest belief in a religion. In our case, we both believe in God. We can pray together, and we do it several times a week. You cannot lie to god, so a common prayer is also a very honest talk to each other.
The base can also be something else, but it should not be just a common hobby.

Have regularly meetings

We have two regularly "meetings" in a week. On Wednesday evenings we usually go for a walk. There we talk about daily things and work that hast to be done. We plan the next week and the holidays. It is a kind of company meeting and sometimes we even create a todo list.
On sundays, we talk about us. We talk about our feelings and our friends and whatever we want, except of planing the week. So this time is reserved for quality- and soul-talk. It is important to find these times, where no daily struggle interrupts us.

Work for your love

When everything is fine, then both partners go their way. This is OK. Everyone needs his personal periods, seasons for their hobbies or to think ones own thoughts.
But there is an automatism. If you don't invest energy into your partnership, the ways you go will diverge. This can go for weeks or months and none of you recognize it. And then suddenly you feel alone. If you are sensible enough, then you have the chance to turn the wheel back, but the longer you wait, the more difficult it is to come back to each other.
If you have the feeling, that your partnership is just a partnership of convenience, then interrupt!
Do something that surprises your partner: For example give some flowers, invite him/her into a theatre or cinema or meal outside,…
Take care, that both of you get back to your love and both of you get satisfied in their personal needs (see next chapter)

Be aware of the different personal character patterns

Everyone is individual in the way of perception/cognition, feelings, thoughts and reaction. Everyone has it's own basic needs. So it is for example possible, that she needs at first appraisal, but he needs body contact. So if both "have a good time in bed" it could be, that his world is well again, but not hers, because he still doesn't give her the necessary attention.
Another trap is, that one partner triggers the other by making "harmless" comments. He/she doesn't understand that a word, that for him/her is harmless initiates a program in the partner.
So it is very helpful to find out who you are. (This is also helpful to get along with colleagues and friends,…)
There are some good books on the market (The "colours of love", the "Enneagramm",…). It is a good idea to read some books about our psychologies and talk about it with your spouse.

Spend some extra time

Now and then you should take one or a few days off and have a dedicated partnership time. Have a weekend on the beach or in the mountains. If you have children, then, depending on their age, you should take them with you, but even then try to find some hours were you can be alone.
Surprise your spouse with a nice evening outside.

If there was a bad event

Sometimes one or both of the partners do something, that is not good for a partnership.
You don't need to tell everything. Often it is more about the "how" you are telling something, then the "what". But you are not allowed to lie. Because if the partner cannot trust you, then you really have a problem.
Be aware of the words you use and the triggers of your partner, which brings him/her into that mentioned psychological stress program. Try to avoid this, because then you cannot talk logical. If this happens, interrupt your discussion and go on later.
If necessary take help from your friends, church (they are usually very good in this and often offer their help for free) or "professionals".
We also went to a partner training and it was very helpful. And it is nothing to be ashamed of.

So I hope I could save some marriages with my post ;-)

Regards, Achim