What It Feels Like to Grow UpAgeing

"Feels like" is specific to emotions/physical sensation; you talk about more than just this.
"Growing up" is only used for children, and after that, you're considered (a) grown up, so I think "ageing" is more appropriate for your piece.

Lately I've been thinking about the meaning of growinggetting older.

"Lately" is a bit redundant here - this is implicit from the tense.

It's not only abouthe things that othe external sidr people see, like changing homes or making important decision; it'ss, but also about the insideyourself.

Good use of the semi-colon - this would be a great sentence if "external side" were used in this way. Unfortunately it isn't, and I can't find a satisfactory substitute, so I have changed the structure a bit to use more typical vocabulary.

Sometimes I feel like everything is moving faster than I can process.

Perfect.

There are days when I'm excited about new beginnings, and others when I feel a bit lost, as if I were'm still searching for my place.

I had to think for a very long time about this one. You're correct that "as if" was historically always followed by the subjunctive "I were". However, this structure can only be used with a counterfactual - that is, when the following statement is known to be false. Here, it is not necessarily untrue that you are "still searching for [your] place", so this construction cannot be used. In modern usage, "as if" can be used without a counterfactual (like here), but in this case it never takes the subjunctive; hence "I'm".
Sorry - this is a very technical grammar point. Let me know if you are confused. To be honest, in modern English usage, the subjunctive is often totally ignored - so if you're not totally sure, you can just omit it and no-one will mind.

I've also noticed that growingThis process isn't always comfortable.

Good to avoid repetition - and "I've noticed" really should be followed by something non-obvious or profound, which (unfortunately!) this is not.

Sometimes it involves letting go of expectations, facing feelings I'dthat one has left aside; - or accepting that some relationships change.

Really nice. The "that" can be omitted, but I think it brings necessary clarity here. You would need the perfect tense here ("feelings I've left") as the feelings are still around; as you're talking about "growing" as an abstract, general thing, you shouldn't use the personal pronoun "I", instead using "feelings you've left" (more common in modern English) or "feelings one's left" (more interesting and technically correct). Finally, the semi-colon should always replace a conjunction, and if you want a "longer comma" for dramatic effect, use a dash.

But at the same time, there are little victories that remind me I'm moving forward: understanding myself better, making decisions that used to scare me, or simply feelbeing more honest with myself.

This is a lovely sentence with no grammatical mistakes. The use of a triple (a list of three points) is a common device in English rhetoric. However, I think it could be improved further by trying to eliminate the repetition of me/myself - for instance, you could write "a better self-understanding, overcoming fears and being more honest with myself".

I believe I'm in a stage where many things are changing at the same time, and although that confuses me a bitis is scary and confusing, it also gives me a sense of strength.

Don't beat around the bush - it's your writing, so you can present what you think about yourself as fact. Try to avoid repeating the subject: you're clearly talking about yourself, so you can say "this is".

It'sn a weird, but beautiful. way,

This is a bit too cliché for my tastes, but if you want to keep it, the grammatical structure I suggest eliminates the need to start two sentences with "It's".

It's like'm learning to live withbe a more sincere version of myself.

"Living with oneself" is generally negative - as though you are tolerating a bad quality in yourself. You are doing the opposite here, so I prefer "being".

There are days when I'm excited about new beginnings, and others when I feel a bit lost, as if I were'm still searching for my place.

This isn't a correction. It's a question: If you feel like you are still searching for your place some days, then the present tense may be better because everything else is described in the present tense. The consistent use of that tense gives the description of "what it feels like to grow up" its immediacy.

"As if I were still searching" means that you are not, in fact, still searching. It means that you have found your place, and you know that, but there are days when you feel like you are still looking for it. If that is the meaning you intend, then the verb tense supports it.

Lately I've been thinking about the meaning of growing (up?).

Just to make it match the title. "Growing" on its own suggests a more physical concept than "growing up".

It's not only about the external side, like changing homes or making important decisions; it's also about the inside.

I've also noticed that growing up isn't always comfortable.

Sometimes it involves letting go of expectations, facing feelings I'd left aside;, or accepting that some relationships change.

Semicolons are useful, but this is a list, so I would suggest sticking with commas throughout.

I believe I'm inat a stage where many things are changing at the same time, and although that confuses me a bit, it also gives me a sense of strength.

Nothing wrong with "in a stage", but "at a stage" feels a little more natural when talking about stages in development, life, a process, etc.