It also helps to increase available countries where I can speak with their languages.

But there are downsides- like learning more slowerly, so my English skill ,which is my primary goal to speak fluently in, isn't good enough.

About ten years ago, I met a perssomeone who could speak more than 6 languages.

'A person' is perfectly fine, but 'someone' sounds just a little more natural.

I had a fixed mindset that I must learn only one language at a time.

ByWith his advices, I could break my fixed ideawas able to break away from this mindset and I have been trying to learn three languages.

'Advice' is always singular, not plural.

'Was able to' is better than 'could' here. 'Could' sounds like you were able to, but didn't.

'Break my fixed idea' sounds odd. You usually don't 'break' an' idea'.

'Fixed idea' sounds too formal here.

On the plus side, I think it is more interesting because I can switch learninganguages when I am bored.

It also helps to increase available counties where I canbe able to speak with their language in more countries.

'to increase available countries' sounds a bit odd. It sounds like more countries are being created because of you learning languages.

But there are downsides, like learning more slower, soly, which means my English skill, which is my primary goal tofor speaking fluently, isn't good enough.

When using 'slow' as an adverb, it becomes 'slowly', not 'slower'. 'Slower' is for comparing.

'English skill' is fine, but it's probably more natural to just say 'my English'.

Still, if I sticking with it, I can continuousgradually increase my speaking abilities.

'Continuously' sounds a bit odd here, even though I understand what you mean. Gradually might be better.

It sounds more natural if you rephrase it with yourself as the subject. (For example, 'if I stick with it', instead of 'sticking with it')