Learning multiple languages at once
I'd highly recommend you study one language of a family tree at a time. Like one Germanic language and one Romance or one Slavic language. Or maybe add in Hungarian, Japanese or whatever other language in too. It is really easy to get mixed up with languages of the same family. So until you are about amateur or a proficient speaker in one language of a tree, don't try tackling the other languages in the tree just yet.
I was doing Swedish and German at the same time and getting very frustrated because I kept mixing up basic terms. I'm going to do a little French on the side of my German just to learn the basics of French pronunciation because it is common in some books I read and phrases and I just want to learn the basics of French. And once I get better at German I may pick up another Germanic language.
Of course, if you are really good at learning languages and dont mix it up, ignore what I said. But just an idea. Good luck fellow language learners
On the other hand, the similarities between languages can really help you strengthen both. It's really great to ladder languages from the same language family, because that points out both difficulties and similarities. I find my problem is mostly with spelling - often words that are pronounced pretty much the same are spelled differently, and remembering which version is used in which language is difficult. Elephant, tea, and coffee and other words that were borrowed by many different languages are a big problem. And you can mix up words from different language families, for instance, Je in Dutch is you, Je in French is I. Since they're both pronouns, they're pretty easy to confuse, especially when you're challenging yourself to go quickly.
The Dutch 'Je' has really been doing my head in - I can't count how many times I've translated a sentence as if it was written in the first person (after learning French at school thirty years ago), only to have it marked as incorrect!
Since I learned French and Italian independently from each other in the past, with ten years in between, it is easy enough for me to distinguish the two, and learn new skills in both. But I've realised that there is no way I'd be able to learn Spanish before being a lot more fluent in Italian. Dutch is distinct enough from both for me to learn it at the same time, and since German is my native language - together with English - I don't have to worry about learning all the ins and outs of the V2 word order. But learning Dutch also means that I won't be able to learn Swedish until I've reached some degree of proficiency. Which leaves me wondering which language family to explore next ...
I was considering Russian, since I've been fascinated by alphabets since I was a child, and I've been familiar with the Cyrillic alphabet for decades. But I'm still not able to read it fluently - I guess that comes when you start learning words, and you see them as a whole rather than the individual letters that they consist of.