How to switch languages quickly and easily?

Basically no problem. What you are describing is what I call "linguistic interference." I run into that problem too , it's rare, still it does take place between French and German.

My solution (far easier said than done) is keep practicing in the language , pound away at it, which includes practicing writing it properly (the key word here) until you have internalized it, flipping between it and English almost instantly, thinking in the language.

but I only seem to have one slot in my head for one foreign spoken language at a time

I speak six languages ​​myself, and if you were to put six people next to each other, each speaking one of those languages, I would answer them one by one in their own language.
The only difference might be the speed of the conversation which might be a bit slower in the languages ​​I use the least or that some answers might include less complicated words because the amount of vocabulary known varies somewhat between languages. And of course, some conversations may contain more or fewer grammatical errors.

The blocking effect that is overpowering you is probably psychological. The fear of failure, being misunderstood, and making grammatical errors are the three most common reasons why people are afraid to switch to another language. Making mistakes feels like failure. And that seems like the "end of the world" to us.
Then there's the inevitable fear. Fear reduces your brain's ability to function properly. This means that if you're afraid to speak another language, your brain can't process the language properly.
The fear of being judged is usually present as well. Most of us are afraid of being judged. The same applies to foreign languages, because we often think others will judge us for our grammatical or pronunciation errors.

I know what you mean.

I speak two languages, and in school or in life studied five more (none of those five can I speak with any fluency, though I have phrases in all of them or random verb conjugations, vocabulary in a few domains, or whatever).

But there seem to be three buckets in my brain.

One that is labeled "native language" (English) and that I can speak without thinking about.

A second one that is labeled "French" - I can prerry much pull out of this one what I want. Not 100%, but enough to live and work and do my errands.

Then there is that third one, labeled "foreign language." This one is the problem. If I am in one of these countries, my brain knows it will need to reach in here for what I need.

But !!! Everything in there is all jumbled up. German is as foreign to me as Italian which is as foreign to me as Hungarian which is as foreign to me as Russian which is as foreign to me as Spanish. So when I tell my brain "it's time to use a bit of this foreign language now," I am as likely to come up with old Spanish phrases from high school as something from the Italian news i watched last week. I will think "puedo" is a darned good substitute for "posso" and not understand why my mother-in-law is looking at me perplexed.

That bucket just has way too many incremented pieces in it. I reach in, hoping to get something appropriate, but it's absolutely hit or miss.

One that is labeled "foreign language" and to which I dip in (undoetunatelh

Quickness comes from use and exercise. In the Alps there are several areas that use two or more languages in everyday life - think Tyrol (the Italian section is German/Italian bilingual, actually trilingual as there are Ladino areas; but also the Austrian Tyrol as a lot of Italian speakers) or Switzerland (four official languages, and Romansch has several dialects almost mutually non understandable).

In these cases I have often heard periods beginning in a language and ending in a different one; or single words may being lent from one language to another (for example, Swiss Italian speakers say "Azione", from the German "Aktion" for a premium deal in a shop, while the standard Italian is "offerta speciale"). The extreme case was when I stood listening for several minutes a supermarket cashier in Disentis, Switzerland, interacting with her clients, without being able to realize if she was speaking Romansch or German (two widely different languages) as everything was throughly mixed.

Use and exercise in language comes from necessity. If you have to deal with a problem you will have a powerful reason to speak in the language of the problem. In time, you become multilingual.

I absolutely know what you mean. I seem to only have room in my brain for one non-native language at a time. I think it gets harder the older I am. I think I really focus on ____ language in the time leading up to and during the trip so I kinda purge my foreign language memory banks of everything else temporarily.

And traveling from Italy to Spain - one would think it was easy, but I end up speaking the wrong language half of the time.