I have the same problem. I was talking to some French friends recently and every so often I would notice a blank expression appear on their faces and realise that I had lapsed into Spanish. It doesn’t happen so much the other way unless it’s a word or phrase that I know without thinking from French.

I recently bought a French/Spanish dictionary and started looking up words that I know well and use a lot in one language to see what they are in the other language. It seems to me that there are trigger words or phrases that make you switch tracks and it should be possible to neutralise them by learning their equivalents equally well.

Some of my common triggers are siempre/toujours, il faut/hay que, il y a/hay and no matter how hard I try the odd oui or si tends to pop out at the wrong time.

If you are into NLP techniques you could try using a visual method to separate the two languages. Without being too analytical ask yourself the question “where does French live?” your eyes and or attention will want to go in one direction. If you want to stay in French mode in future, keep glancing in that direction.

I just tried that exercise with each of my languages: English took up the whole of my front view about 160 deg, French was slightly left of centre maybe 5 deg wide and Spanish slightly right of centre and slightly wider than French. Let me know if that works for you …