I agree with you that relationships are far more fullfilling then friendships - I think most older and married people would. For most people there is a clear order in their social interaction: Close family, Close friends/wider family, casual friends/distant family etc.
I don't deny that a close friend cannot become as close as a family member, but for the vast majority of people your partner will be the closest person in your life, only rivalled by your kids parents and siblings.
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But, its not healthy to only have your partner. I have lived like that. When I first moved coutnry, all I had was my wife, and I could barely speak the language of anyone else.. its not healthy for your relationship at all - it puts way to much pressure and demands on each other, and it can cause serrious problems in the long-term. You don't need to have a lot of friends, but just having 1, or a couple of other people that you can see outside of your relationship will help you a lot.
As for how to meet them - the easiest way to make friends for people who don't find it easy has always been the same thing: join a group based activity, where you can socialise whilst doing soemthing. It takes the focus off you - and lets you slowly make friends with people you will see each week, and do a fun organised activity with each week.
26 is not to old - I am older then that, so is my partner, and when we move country again next year, we will have to go through the whole 'finding new friends' thing again. Infact I would say as you get a bit older, it gets a bit easier. There is a bad patch after the normal 'university age' where people find it hard to make friends.. but as you get into your late-20s and early 30s, people start to accept that they are older, and actually need to make an effort to make friends.. it won't just happen naturally like at university and school, so more and more social oppertunties are forced and open up.