Esperanto is not a useless language and it is asinine to claim it is.
If English is your mother tongue German is somewhat more challenging than languages such as French, Italian, Portuguese or Spanish. It is also a little more difficult than Dutch. However, the key to learning any language is having the desire to do so. It is also important to practise it regularly.
I can speak Esperanto and it was a great help in getting to grips with language concepts that are not taught to native speakers of English in English language classes. I am learning German and am enjoying the challenge. I practise every single day without fail. I am also fortunate that I can get to speak with a (non-native) fluent speaker of German on a weekly basis. I think that is of great help. Nowadays we put a lot of focus on the written word but languages are meant to be spoken so getting practise at speaking it is vital.
It depends on what other languages you speak. If you speak any of the Germanic languages, it’s fairly easy. If you speak Gaelic, the pronunciation is a doddle. If English id your native language you’ll have some difficulties with the genders (but you’ll have that in most languages), and with the pronunciation. You’ll be able to guess a lot of words (~30%) without having to learn them (e.g. Der Doktor sagt, ich muss am Arm operiert werden - The doctor says I need to be operated on my arm) (compare Ich muss - I must (/need to/have to).
It is not as difficult as non European languages but it is not easy.As far as Esperanto is concerned it is only useless if you have no other people to talk with.Do not use it as a stepping stone to learn another language.
I found it very difficult, whereas I found French and Spanish relatively easy.
I can make myself understood in German, but I still get the 'der, die, dem, das', etc, mixed up, as well as the word order in sentences.