It seems easy to know yourself, but actually, it is no more difficult than anything else.

Using the phrase 'but actually' is usually followed by something that contradicts the first statement. Since you said it 'seems easy' the second part needs to be changed to make it clear it is actually hard.

No matter how long you look into a mirror, all you canwill see is a person who does not know themselvesf at all.

'themselves' is plural but the subject is singular so it needs to be changed to 'themself' (Or alternatively 'himself' or 'herself' if you want to use a gendered version)

Or rather, the state of mind to findyou have when looking at yourself in the mirror may make it difficult to know yourself.

Based on your Japanese sentence I think what you are trying to say here is that the feeling you have while looking at yourself in the mirror is a hindrance to finding yourself. If this is the case then the first part can be changed as I corrected to express that.

It seems easy to know yourself, but actually, it is no more difficult than anything.

Or rather, theyour state of mind to find yourself inwhen looking at the mirror may make it difficult to know yourself.

It seems easy to know yourself, but it is actually it is no more difficult than anything.

No matter how long you look into a mirror, all you canwould be able to see is a person who does not know thehimselvesf at all.

Or rather, the state of mind required to find yourself in the mirror may make it difficult to know yourself.

I am not sure what this last sentence is trying to say, but have assumed that you are saying that the state of mind a person has when looking in the mirror makes it difficult...?