Because you are detoxing.
When you give up the bad-for-you-things, whether they are physical or emotional, your body and your mind rebels.
If you are an avid coffee drinker give up coffee and see how you feel, or stop smoking, or eating sugar, or start exercising after a long period of inactivity. Usually it's a huge pain in the ass.
Then look at yourself two or three months later. You'll be freakin' rocking it.
That's the nature of change and transformation. It's painful.
If you want to be happy start thinking happy thoughts and it will happen. That's usually a big pile of b.s. sorry to disappoint you. If you are the most negative person on the planet and you decide that you want to change all of that and be positive you have some work ahead of you.
First you're going to have to deal with the people in your life. They're going to be used to negative, glass-half-empty-you, and chances are, they're probably the same kind of people. So you turn up happy and positive and they're going to be very confused or resent you and not be happy for you. Which is going to bum you out and make you want to go back to negative you.
What are you going to do the first time something doesn't go your way? You get a flat tire on the way to work and forget your lunch and get in trouble for being late. How are you going to deal with that? You'll probably roll your eyes with an "Of course this was bound to happen because nothing ever good happens to me so why do I even bother?"
Positive change takes a lot of work, but once you get the hang of it, it's awesome.
Everyone has a certain set of default reactions, feelings, emotions, and coping mechanisms inside of them. We can change them, but it's not easy and it takes more than a few days, weeks, and sometimes even months and years.
It takes time for those changes to stick. The problem with New Year's Resolutions is that people expect it to be easy, they expect the good things to click right away and to override the bad or negative habits that are the default.
If it were that easy, then we'd all be perfect.
If there's something that you're working on, something good-for-you, and you keep seeing all of this negative gunk coming up in your life as a result, acknowledge that you're detoxing. It's allowing you to get it out of your system, to deal with it, or to learn new ways to deal with it.
It doesn't mean you are failing.
It doesn't mean you should stop.
It just means that you need to keep going, keep leaning into it, because when you really come out the other side. You're going to be so damn proud.