You got unlucky. That’s why I wrote in my post, “I got extremely lucky”, because I am not arrogant enough to say “gee I had such a good eye/instinct” - I recognize it’s often pure luck. Like when you carefully examine lines to cashiers at a store, and somehow still pick the slowest one - often it’s more luck than skill.
That said, I like you, watched friends and their relationships, and perhaps - bad luck for them, good luck for me, some of them had really bad ones. And I tried to observe what caused such failures. There are of course as many reasons as there are bad relationships, but a few things stuck out to me. A huge one which a friend of mine was really prone to: the woman is extremely attractive or sexy and as a guy you are all in on that aspect to the exclusion of everything else… sounds banal, but you’d be surprised how often folks make that basic error. Like my friend - over and over again, hot girl, then a year later, disaster. So I said to myself - yep, you gotta have some basic attraction (or it won’t work), but you really, really, really have to focus on “but can I live with this person? Do we have anything to talk about? Are we compatible? Are there any deal breakers?” Because if your partner only shines at night, briefly, then that’s really not enough for a long term relationship. That saved me more than once.
MIL - well, that in turn, I think is pretty simple. I’m not a believer, but there is some hard won folk wisdom in the bible, and it does say something to the effect that in marriage a partner leaves his/her family, and cleaves unto their partner. In other words, you LEAVE your family and JOIN your partner in marriage. Doesn’t mean you can’t have a relationship with your biological family, only that the marriage is the PRIORITY. Your partner comes FIRST before anyone else, including your family. Many people don’t believe that, but I think that’s the foundation of marriage - if there is someone more important than your spouse - family, religion/religious figure, etc. - than that will inevitably win over you, and f.ex. your MIL will be more important than you, because she ALREADY is, before marriage. But how do you know ahead of time? Well, you know how teenagers talk - every few minutes you’ll hear “my dad” or “my mom”, parents are such a big part of their lives, it’s only natural. But if your date is 25, or 35, or 45, and they still reference their “mom”, “dad”, “sibling” constantly, then they have not cut the apron strings and are just in an infantile relationship to their family. You’re grown up. You don’t have your parents live in your head 24/7. Doesn’t mean you must abandon your family, just that you must be your own person, not rely on parents for your mental health. A healthy relationship to family is when you can say “love my family but I’m living my OWN life, I have flown the nest when the time came”. Those who have not launched are trouble. My rule of thumb is if somone on a first few dates can’t stop talking about their parents it’s a bad sign - also, if a man/woman talks to their “mom”, or “dad” daily on the phone, or constantly runs to them for every little thing - GOODBYE. They’re mentally a child, no matter if they’re 20 or 50. And a tip - may not be practical - marry someone from a different continent. My wife is European, we live in the U.S. - very good relationship with family, they visit us, we visit them all according to choice and convenience. Never LIVE with the inlaws, best, separate by continent - OK, I’m joking a bit, but hey, it’s worked for me.
Finally - and now I’ll get shouted down and get people really angry at me. My apologies ahead of time. Younger is better. Not because of anything physical, but because as a friend of mine said “their stories are shorter”. Didn’t have time to become embittered, which they inevitably may take out on you; or you get compared; or they become a bit cynical or wary; less flexible; too set in their ways. Now, don’t get me wrong - nothing wrong at all dating older folks - it can be fantastic… it’s just that you are really upping the bar for “luck” here… you must get that much more lucky to find the right one… it’s possible, just harder.
And of course, this should go without saying, but - be the best husband/wife you can possibly be. Because it takes two. If you are fantastic, they’ll be their best and the marriage has a better chance to be a success. The better YOU are, the better the marriage. Don’t only look at “what can he/she do for ME”, but “what can I do for her/him”. Like JFK said in another context “ask not what your country can do for you, ask what you can do for your country” - it’s exactly the same when it comes to your spouse. Be generous in every way, emotionally, spiritually, materially, intellectually. Nourish them, and they’ll nourish you. And if despite that, things don’t work out, you can dissolve the relationship with a clear conscience, you did your best. Don’t stay in a bad marriage/relationship. It aint’t worth it - you got one life and you’d be a fool spending it unhappily for whatever reason if you can help it. “Darling, it’s not worth it, let’s move on! Ciao!”.
Best of luck - because I ain’t gonna lie, luck is a HUGE part of it. YMMV.