The road to multi-lingual

The reason why I've started this blog in English, is because I am scared of losing my English, or rather somewhere in my brain, I wonder if some English words I knew have been replaced by similar French words as I learn.....

There are heck of people who speak several languages - and I don't even have enough capacity to store more words in just one new language?
That's not fair!

Believe me, after living in England for 12 years though, surely I am losing my English and it's very quickly. Despite the fact that my husband and I speak in English most of the time and watch BBC or CSI in English. (Although this doesn't help with my French study at all - can't be helped)

The way I speak is getting a bit more aaaaah...f..French...iish?
Nope.
No way near it.

It's been over a year since I started to learn French properly.
These days, when I speak with my husband, I notice quite often that I mix some French words in English context.
When I study French, I'd think things in English - that's easier for me as there are many similarities. But
When I respond - I'd answer 'Oui', instead of 'Yes' - that's a small example.
Instead of saying [I want something little] I'd say [I want something petit] - something like this.

Not realising what language I was using....that's the problem.
Yes, of course, you'd speak something normally - but I should be more careful not to be lazy and stop mixing the two languages.

I'm very curious to know how the brains of the multilingual people works - does anyone have any idea how to sustain what's in the brain and increase the memory capacity?
It's very long way to go.....am getting old.