I would like to give you some practical advice on what you can do if this happens to you.
1. First of all, don’t panic. It does not help. Make a plan how you will use the time available in the most effective way.
Be realistic. You can’t learn English in two or three months but you can improve a lot. You can activate your passive knowledge. You can get into the practice. You can refresh what you learnt years ago. You can learn basic vocabulary to survive in a foreign country or just to pass an exam.
2. Be focused. You must practise what you really need. Is it a job interview? Then practise that with a private teacher every day. Brainstorm what you may be asked in the specific situation. You can’t learn everything but you can prepare for a specific situation.
3. Deal with it as much as you can. Learn English every day, every hour, every second. Live with it. Sleep with it. There is no minute to waste. If you are lazy, you may miss your goal.
4. Unless you need to learn general English, it might be better to get a private teacher who can focus on what you really need. A language course with more students could be cheaper but you will not have the opportunity to speak as much as you can in a one-to-one lesson. Moreover, in a pre-planned course with others in the same group you can never have that tailor-made approach that is the advantage of a private lesson.
5. Use technology. We live in the 21st century and that has a lot of advantages and solutions that we can exploit. You can download materials from the net. You can put listening texts and dictionaries on your smart phone. You can have online lessons and you even save travelling time and learn English instead. Common sense. Use these techniques.
6. Never get disappointed. You will probably feel sometimes that you are not making progress. This is always part of the language learning process. If you put maximum energy and effort into your project, you will achieve a lot. This is obvious. Never doubt it. So think of your goal every day and do your best. There is nothing else you can do anyway.