Outside of death and taxes, the one thing you can count on in life is change—more specifically that people will change. And while you may know intellectually it's inevitable, when the change happens to a partner or spouse, it can be emotionally shocking. After all, this new person isn't the one you fell head-over-heels in love with, and they're most definitely not the ones you said your vows to. So what do you do now? Your options are as basic as 1, 2, and 3.
1. Try and get them back to their former selves
This, of course, is probably easier said than done, but if you feel compelled to try, then at least proceed with caution. It may be that your partner actually wanted to change and likes the new them, in which case you'll likely be resented for trying to get them to "go back". However, if you really believe your guy or girl to have changed for the worse (i.e., drug or alcohol addiction, self-destructive behavior, etc.) then you'll at least want to give it a shot.
To start, make sure you don't sound like you're motivated by anger or that you seem threatened in any way. Tell them that you've noticed what's been going on and you're simply concerned for their well being (this works especially well if there's been a weight-gain, or if they seem to have let themselves go in some way). Do not make any threats to leave—even if in the back of your mind you're considering it. It will only make them feel as though you're against them and not to be trusted…and they'll be less likely to go back to their former selves.
Also, if you can, try and get to the "root cause" of the change. If your ladylove has suddenly ballooned, it's not likely that it's just because she really likes pie. More likely there are emotional or psychological reasons for her overeating, and working on those is really the only way to get her to go back to her slender self for good.
2. Stay and live with "the new them"
Okay, so maybe you've already pulled out all the stops and you can see there's just no going back. They are who they are now, and you need to either love 'em or leave 'em. If you're going to love them, it will take some doing—on both your parts.
First, if you've made the decision to accept them as they are, you cannot continue to criticize them, covertly try to move them back in the other direction, or cheat on them to fulfill your own needs.
What you can do is find common ground—by rekindling long-forgotten passions or creating new ones—to keep you connected. Try and remember what it was that brought you two together in the first place, or if that passion is long-gone and impossible to revive, make an effort to explore new avenues together…whatever those may be. This is a great first step toward redefining the relationship and creating a shared picture of its new future.
3. Hightail it out the door and leave
Though it may sound cold, there are some times when you just gotta look out for No. 1. And if you're not happy with your partner, you know they won't change, and there's just no bridging the divide, it's probably time to walk away. In other words, you gotta know when to hold 'em, and know when to fold 'em. Just make sure that before you decide to fold 'em, you did all you could to salvage the relationship. You owe it to your partner, yourself, and the relationship to walk away without any lingering "what-ifs".