The hardest language to learn depends on what languages you speak.
My native language is English and German is the only other language I speak.
There is no such thing as an „easy“ language to learn. It’s often said English is „easy“ but I can’t tell from experience because I never studied it. But, since English dominates this board by far, all you have to do is peruse the answer and discussion boards and read what non-native speakers write and you’ll see English is far from „easy.“ I think all foreign language learning is difficult, whatever the target language might be. And, of course, that relates to which language you start with, and is a variable. This is because of the massive vocabulary, it’s proper uses, and the idioms of every language that have to be matched with with corresponding idioms in the target language because they can’t directly be translated (except on rare occasions). German grammar is not harder than English grammar, in fact German grammar is easier. The reason English native speakers think German grammar is difficult is because they never learned English grammar in the first place. They just acquired their language the way all children acquire their naive language. It’s a biological phenomenon, not well understood, but just happens. I started learning German in 1977 and within two years of constant courses and study, we had covered all the grammar there was to know. That was a total of 300 hours of classroom instruction. By the time I went on the upper division German classes, that required 2 years of coursework ahead of time, the classes were taught by native speakers in German only (the prior classes were mixed) and once the conversation started, with reading books in German, my fluency skyrocketed. It was truly amazing. With the grammar as the basic groundwork, the vocabulary building was all that remained. And acquiring vocabulary is a formidable task. The grammar is easy. Vocabulary building is hard in any language. That’s why I say none are „easy.“
Bing, don't give it up yet!
Learning a language takes time an dedication. German can be really difficult and I know what struggling with its grammar feels like.
You can try different methods to "spice it up". Deutsche Welle has many free resources to learn German. I particularly like Nicos Weg, as it covers contents from A1 to B2. Goethe-Institut has a page called Deutsch für dich, where you can chat with other learners. Besides, you can try Duolingo to learn vocabulary, Assimil to ease yourself into the grammar or Pimsleur to get more confident speaking (the last two options are not free, though).
As for the most difficult language for me... I guess it's Spanish, because I always mix it with Portuguese and end up speaking Portuñol. Besides, as it's spoken in many countries, the accents are a little bit confusing.
Good luck in your language journey!