Tamara, that’s annoying. Everybody wants to show that he knows English or wants to improve it. Please do persist in trying to speak Catalan and explain this to the people.
I suggest that you find a language partner English/Catalan and that you engage yourself at having daily conversations. Start to think in Catalan, when you get up in the morning and prepare your breakfast comment everything you are doing in an inner speech and so on.
You've really thrown yourself in the deep end! I found it was easier for me to manage in Catalan than Spanish in Barcelona, as there are more cognates with italian and French in Catalan.
I realised there were also cultural issues regarding using Spanish or Catalan in that area (several of my contacts spoke Spanish but were not from Catalonia) so you need to take that on board.
If I were in your shoes, I'd arm myself with suitable phrases such as "I'd like to speak in Catalan" or "How do you say that in Catalan?" and use them a lot. I'm sure you'd find a few people who are happy to guide you. Practise your puppy-dog eyes. XD
Following on from Aliph and Peachy’s excellent advice, you might invest in a frequency dictionary of Catalan and try to actively learn the 500-1000 most common words in the language. Better still incorporate them into useful phrases and you’ll have a solid base to work from.
When I arrived in the Czech Republic as an English native I had the same problem. I organised as many language exchanges as possible in order to have longer conversations , during which I knew I wouldn’t be ‘cut off’ by people reverting to English.
There was no Italki in those days so if you can’t find native speakers to exchange with, then book some lessons here - it would get your language moving quickly and would motivate you more, perhaps.
Try not to be too frustrated or unhappy or recognise that what you are experiencing is completely normal. Fluency will not happen overnight but consider your learning as a long-term project. Best of luck!