In my experience, it was at times confusing trying to learn two very similar languages. I could manage improving my Swedish and Spanish at the same time, but trying to handle Spanish and Italian was more difficult.

However, learning Spanish first and then putting it on the back burner in favor of Italian worked quite well. I could then concentrate on learning new things in Italian, while being able to move forward faster on some grammatical areas which were already familiar from learning Spanish.

Since we're all different, I think it's best to experiment with different methods and see what works for you. Sometimes being able to take a break from your main learning language is useful, too, to keep up your motivation and to feel like you're making progress.

Hope this helps!

I started studying German when I was feeling demotivated with my French, and the happiness of learning new words and things gave me more energy to dedicate myself to French again.

Still, there are ups and downs and you may not be able to think properly in one language, have the impression that you're not making progress or something like that. To avoid that, I usually focus on one language while I study other/others at the same time. For example: currently, my priority is German, but I still chat in French or watch something to keep it fresh. When my German is better and I feel more confident, I am thinking about giving Spanish or Italian a go.

But it's just how I deal with it and people have different styles, motivation, etc. Anyway, good luck!

Thank you all for the comment! Some tips here as well:

 https://www.youtube.com/watch?v=MdZnkatthxg&t=340s

Thanks Timo!

I do agree, It's very confusing to learn similar languages at the same time . 

Motivation is everything!
All the best,
Zsolt