Bit of an off-the-wall suggestion but I was having real trouble learning other languages mostly due to all the cases (and sometimes genders). So I started learning Esperanto, which admittedly is not the most practical language to know BUT it really helped me understand languages more in general. It’s very simple to learn and easy for an English-speaker to pronounce, as well as having lots of recognisable words (the Esperanto for “yes” is “jes”, which is pronounced “yes” lol)

Now that I’ve gone back to learning German I’m feeling much more confident about learning in general just because I got some of the grammar concepts from a much easier language. Might be worth a look if you’ve struggled before.

Spanish: it’s the most return per effort.

Esperanto is reckoned an easy pickup, has speakers globally, and will improve your default in most romance languages. The community is also quite nice, in my experience.

Not OP, but I’ve asked myself this as well. I think it depends on where you live and what you want out of your language learning experience. If your goal is to learn something more useful in everyday life and you live in the southern US, Spanish is a great option. If you’re from Canada, French is probably the most useful. German and Mandarin are useful in the business world, but the latter is significantly harder to learn. If you’re not worried about maximizing the utility of what you learn, Norwegian is considered one of the easiest languages for English speakers, and let’s be real, Norway is awesome.

It’s more important that you stick with whatever you choose though. That’s the part I’ve struggled with.