Who am I?
I'm constantly torn between things. It's like I never have a strong opinion or stance on things. It's like I don't have an identity or values. This has become difficult to sustain mentally.
Example #1. I despise Instagram because of the way it transformed people's behavior and because of its privacy abuses. For these reasons, I refuse to have an Instagram account and I don't participate in it. At the same time, Instagram is admittedly essential in today's social life, so I feel I am missing out.
Example #2. Sometime in the past, I got interested in climate change. I read books, papers, articles... You name it. I changed my lifestyle to better align with the values I had internalized. I started preaching others about the real threat of climate change, about the need for action by everyone. Now, years later, I don't care about all that any more. I believe doom is inevitable, and that there's nothing we can do about it. So I stopped pretending.
Example #3. I've been an open source enthusiast for quite some time now. This means I run open source software wherever I can. This idea got to the point where I refused to use any software that was not open source, much like a vegan does with food. No WhatsApp, Spotify, Microsoft Office, ...basically any popular software that everybody uses. Over time, I got tired, and stopped caring about open source that much. I'm even thinking about going all-in Apple now (yes, one of the worst enemies of open source you can think of).
You should have a general idea of how it works by now. One day I'm a diehard Linux enthusiast, the next I don't care at all. One day I delete my social media accounts, the next I create them anew. Not literally the next day, but there's always an internal conflict or a straight U-turn with regard to what I believe in, or what I stand for.
Especially examples #2 and #3 could be generalized to any hobby, habit or interest I pick up. They eventually fade out or are replaced by other things. Things I strongly believe in suddenly lose their meaning or value.
I don't know who I am, or what I stand for. What's wrong with me?
I'm not even sure I'm saying something that makes sense. Maybe I'm just conflating totally unrelated issues about myself in one single post.
You are change.
"You're under no obligation to be the same person you were five minutes ago." - Alan Watts
It sounds like you try new things with enthusiasm. It's common for people to be more enthusiastic about new things when they're younger, and then be less concerned about them as the age and grow. (You didn't mention your age, so not sure if that's relevant.) There's nothing wrong with that.
I guess I'd ask why it concerns you. You tried not using Instagram, but found it made socializing harder. So now you have to decide, is sticking to a strict moral code more important to you than interacting with friends? Nobody can tell you what the right answer is for you. And you're allowed to change your mind after trying it one way or another, as many times as you need to. This attitude that you're somehow a bad person if you change your mind or your habits is nonsensical. How can you ever try something and know whether you like it or it's something useful for you if you're not ever allowed to change your mind? And if you can't change your mind and you find something isn't positive for you, do you want to do it for the rest of your life and be miserable? I sure don't!
You also don't have to go "all in" in either direction. You can still, say, drive fuel efficient car, but also leave a light on when you sleep. It's not as efficient as turning the light off when you sleep, but it's also not the end of the world. There are much bigger entities driving climate change than you. Feel free to only do as much or little of these things as you like.
You can use instagram, but also use ad and tracker blockers to reduce the amount of data you give to Facebook and their advertisers. It's not perfect, but it's better than having no social life.
There's nothing to feel bad about here. But maybe the fact that you do means you are interested in thinking more deeply about your thoughts and actions. Maybe doing so with a psychologist or other counselor would be useful? Or maybe reading about philosophy or psychology would help you figure out how you most feel comfortable thinking about the world and your actions in it? Or maybe you need to take a break from thinking about yourself at all and just live and be without worrying about your impact for a little while. Not everyone can do all these things all the time. Sorry I don't have a concrete answer, but life is like that sometimes.
I do similar things. You don't want to know how many yoyos and rubik's cubes I've got, for example, all acquired after the age of 23. I also went from devout Christian to raging atheist to lay Buddhist, and even stuck to using Arch on a netbook for two years exclusively.
There's nothing wrong with you, you're experiencing a dilemma that has been debated for millenia, the struggle for authenticity.
I'm going to have to dip into some notes from a philosophy class because there's stuff I'm only half-remembering, but here's my understanding and something I find quite helpful to consider: The issue with the struggle for authenticity is it can be difficult to know what you are. It's incredibly difficult, and some philosophers (Sartre in particular) argue that it is impossible to be truly authentic because we are "abandoned into this world" and have to conform to already existing rules and norms.
Sartre also claims that should one strive to be authentic, you must also grasp that you are inauthentic, the result of your navigation of the world you're born into, and decisions made for you. You have two choices at this point:
Accept who you are.
Reject who you are, and attempt to change your values into who you feel you are.
Most of our current self is the result of repeating the above conditions.
You haven't answered (1) and/or (2) enough times to come to a satisfactory position. It's not your fault, nobody ever actually has. To stop considering the above decisions is to stop growing and changing. As @mrbig said, "you are change."
I don't mean this as a concrete solution to your issue, but having had similar struggles, this is the sort of thinking that helped me out, even before I was aware some old dead European dudes had already tried talking about it.