When I write in English I use "I" a lot. Especially when I talk about something I have done or an experiment or any thing. My English is upper intermediate. What is the reason behind this? Which skill should I improve to solve this issue?
Have you noticed how many I's I have used in this post? :)
There is nothing wrong with using "I" and you can't really use it "too much". A more reasonable concern would be not having enough sentence variety, and what appears to be using "I" too much is just a symptom of that.
You can use -ing verb forms instead of writing the subject ("I") in each dependent clause. Compare "when I write in English" to "when writing in English", and "when I talk about something" to "when talking about something".
The subject ("I") can also be removed from independent clauses by using the passive voice. Compare "which skill should I improve" to "which skill could be improved", and "how many I's I have used" to "how many I's have been used".
Often, reusing possessives like "my" is not necessary. In "rewrite my post to get rid of the I's" you already referred to the post, so you can simply say "the post" instead of "my post".
You can also describe things themselves instead of your experience of them, for example "The water was very cold" instead of "I thought the water was very cold", unless your experience is relevant to the discussion.
Remember, these are techniques to improve variety, and overusing them simply would make it harder to read!