In Spain, we usually post an story on Instagram wishing our friends happy birthday.

- a -> an only when the next word starts with a vowel. Exception is for acronyms

I started looking for photos we have taken together and I couldn't find any accepsuitable photos.

- acceptable but comes across a bit formal. I think 'suitable' fits more for this context
- You use 'any' which in English always implies plural. E.g. Do you have any tomatoes, are there any days free next week, I don't have any brothers etc

However, I saw some ancientold photos of myself in the beach, I couldmust be 14 years old there.

- ancient means like super old so unless you're like 80 more appropriate to use old
- could implies you're uncertain as in I could be 14 but really I have no idea. I think you mean 'must' which in this context means more something like 'I look like I'm around 14 so I'm think I am around that age but not 100% sure'. Note the order of certainty would change as follows 'I was 14 years old in the photo (100% certain), I must 14 years old in the photo (pretty certain), I might be 14 years old in the photo (less certain), I could be 14 years old (less certain), I have no idea what age I am in that photo

In this period of time, I didused to think that I was really strong and in shape, but nothing could be further from the truth.

- 'did think' is technically okay but the use of 'did' adds emphasis that you really thought that this was the case. Where something like this would be used would be if someone said to you 'You didn't think you were strong when you were 14' and then you who are not at all in agreement reply 'No I did think I was strong when I was 14', the use of 'did' here emphasizes your disagreement with the earlier statement. It would be more natural to write something like I used to think because there is no reason for the extra emphasis in this context

Even son, I've changed a lot since that moment and nowadays, I'm really proud of myself because I feel much stronger thaen on that timeI did then.