Beware your language of least resistance
When learning a language, many soon discover that the best way to learn it is by speaking it a lot. The more the better.
This is easy if you have no other option. This is why living abroad and immersing yourself in a foreign language is such an way of learning the language: Not having a choice is a wonderful forcing function for language practice.
When you do have other options, like a second language you share with your conversation partner, things become more difficult. Specifically, things become tricky when at least one of you feels more comfortable in this second shared language. At this point it becomes your language of least resistance.
For example, my girlfriend is Brazilian but speaks good English, and English is my native language (together with Dutch). We try switching to Portuguese, mostly so I can learn (but also sometimes so she can relax), but it's often so umcomfortable that we end up switching back.
Even though on an intellectual level I realize it makes sense to utilize every opportunity I get to practice my target language, on an emotional level I experience compelling resistance. You may experience the same thing. You may feel stupid or uncomfortable not being able to express yourself, you may feel awkward being the bottle neck to the speed of the conversation, or you may feel any other combination of uncomfortable feelings.
What surprised me is not the presence of this discomfort. What has surprised me is how strong the draw always is to fall back into the language of least resistance.
In fact, my framing of this phenomenon may hint at why it's hard: It's not that you're falling back "down" into the language of least resistance, it's that you're repeatedly and consistently trying to step "out" from the comfort zone offered by your language of least resistance into the discomfort zone of your target language.
The discomfort may come in all kinds of forms. For me, I believe it shows up as a discomfort of not being able to express myself and a slight fears of missing out on a deeper connection with my conversation partner. For you it might show up differently.
When I sawy discomfort I'm also not referring to one big feeling identifiable feeling that shows up. It may very well manifest itself like that for you, but it can also manifest itself across many micro-interactions. You might not even be aware of these smaller manifestations occurring until you start questioning why you keep falling back to your language of least resistance.
Admittedly I've seen some people that don't seem to struggle with this. When I worked at FIFA (with a diverse workforce in terms of nationality) I saw coworkers of mine speak their target languages with one another consistently. It probably helps that FIFA encourages its employees to practice languages with one another.
What these people have in common is that (what I would consider) fumbling through a language seems like an enjoyable activity to them. Like a puzzle they enjoy solving on the fly, with their conversation partner providing them with hints along the way. I imagine they're not concerned about not adequately expressing themselves or failing to connect deeply with their conversation partner.
So it would seem that approaching language practice with a light and playful attitude, such as theirs, is a helpful strategy.
An additional strategy I have found helpful is to never take a step along the easy path: Don't spend any energy building bad habits. This is akin to never having tried a cigarette amounting to being the best way to avoid smoking. You cannot miss that which you never experienced. Having smoked and quitted myself, on very rare occasions I still experience a modest craving, which I imagine never-smokers are wholly unfamiliar with.
By never exposing your brain to a positive experience (like expressing yourself adequately) associated with the behavior you want to avoid (speaking your language of least reisstance), you will never have built an emotional memory to long for.
I have applied this strategy to my relationships with several of the people I work with. I've spoken German (my target language) with them since the outset, even though I imagine English would still be our language of least resistance. Speaking German has become second nature now. I don't miss English, nor do I long for it — and I imagine they don't either.
In these conversation I still sometimes miss being able to articulate myself well. But when I do, it's as if I miss it in a general sense — not because I'm longing for something I remember specifically and viscerally. In my experience the former type of longing is less strong and less compelling. In the end, it's easier to resist, and it typically won't lead you to falling back into your language of least resistance.