Hmmm... I think a lot of success in long term relationships seem to be really understanding your own personality and expectations (and a potential partner's) prior to committing, and then working to accommodate both as much as is feasible. Some pairings really are not feasible long term b/c too much compromise is called for by one personality type (it seems like it rarely is equivalent), but those pairings can be very happy in the shorter term or under particular life circumstances.
I never had expectations or much ambition to get married, or even to have a long term relationship. It wasn't that I objected to the concept, I just didn't put effort into thinking about it, pursuing it, planning for it, or anything like that. It helped that I never was interested in having kids...this is a very real issue for women who DO want kids b/c they have to try to balance finding a suitable partner with their job and their optimal child bearing years.
Yet, along came a love affair, quite soon after I was an independent adult, that proved absolutely wonderful and here I am still incredibly happy (and married) almost 30 years later. I can't say I ever felt a moment's doubt at accepting his marriage proposal (though we'd been together about 2 years at that point), and I've never had any doubt at all since then that this is 'my Mate'.
However, that does NOT mean we haven't experienced tough patches where we weren't getting along, communicating well, or just weren't good partners to each other. We have had 3 or 4 of these, usually as a result of coping poorly with outside pressures relating to health, job/money stressors, etc. Only one patch that I can remember resulted from actual internal problems, when we appeared to be diverging int terms of some life goals and had to begin to contemplate a split. At first, these patches were very scary, but we now understand they do occasionally come up and we stay calm and work through them.
Occasionally our dynamic gets soured the point where we've taken short breaks from each other (usually a couple of weeks) to kind of reset our heads and our daily patterns. When we were young and inexperienced, that idea was frightening...made us feel like the marriage was collapsing or whatever overly dramatic response our less experienced selves had. But we've always loved each other, and wanted to work things out, and eventually we've worked it all out and gone on just as happy as before.
I also believe there are multiple good hypothetical potential partners available for most people, so I don't tend to think of my husband as 'the only person who ever could have made me this happy'. On the other hand, life is short and opportunities don't always last. It's helpful to figure out what qualities you want in a partner and what are deal-breakers, so that you can recognize and jump at good available partners when you are in the market or you get lucky.
ETA: I just reread the OP and saw that he? says he sometimes experiences boredom etc that he lets color other things in life. Yes, this definitely can happen, and can confuse relationships. Are we just in a life rut, or are we actually bored WITH EACH OTHER? B/C even if the source of the problem isn't the relationship, the relationship can still be adversely affected (as I noted above). Be forewarned that both life tedium AND life stress (too much going on) can derail relationships and I find both to be inevitable parts of life, but there is no need for it to do so if you cultivate good coping and relationship skills. And (at least in my case) both of these definitely improved over our lives together, so that things that might have caused a lot of disruption in our 20s/30s cause much less in our 40s/50s.