Texting to A a "Longt Time No See" Friend

I would put 'long time no see' in quotes as it's a colloquialism. Also you don't need the preposition with 'texting to'. Generally, people "text a friend" but "write to a friend".

I texted to one of my high school friends that I didn't speak a lotto much after we graduationed.

Again, texted isn't followed by the preposition 'to', but you need it after 'speak', as people speak to each other. High school is two separate words. I would also change 'a lot' to 'much'; this is a stylistic preference.

I am very hesitant about texting my friends, even though how much I missed them. very much.

Didn't make grammatical sense before.

I am not a talkative person and it is hard for me find a subject to speak. about.

Speak is one of those verbs which cannot stand on its own. So it must be followed by a preposition. (Also, it is okay to end an English sentence with a preposition despite what some people might say.)

I mostlyMost of the time I'm like, what will I speak about with them?

The sentence reads better by changing 'mostly' to 'most of the time' and putting this at the front of the sentence. Also, it needs a verb between 'I' and 'like': 'I'm like...'.
Finally, 'speak about' makes more sense.

I am better at speaking with a stranger that I won't see again.

The word stranger needs 'a' in front of it. However you could also say 'I am better at speaking with strangers...' Either way is fine :)

Maybe, I should try to text my other friends.

Not necessary to have a comma after the adverb.

I texted to one of my high school friends that I didn't speakwho I haven't spoken to a lot aftersince graduation.

In English, you don't need a "personal article", you normally just go straight to the pronoun.

I am very hesitant about texting my friends, even thoughdespite how much I missed them.

I am not a talkative person and it is hard for me find a subject to speak about.

I am mostly like, "what will I speak with them?ay to them?"¶
Or: Often I wonder what I will say to them. (more formal)

I am better at speaking with stranger thats who I won't see again.

When replacing people, use "who" instead of "that"

Maybe, I should try to text my other friends.