In the IELTS speaking test, a quarter of your score is for pronunciation. Many students confuse 'pronunciation' with 'accent'. These are not the same thing!
Nobody expects you to speak with a perfect British/American/Australian English accents. In fact, the examiner will not judge your accent at all. So stop worrying about your accent, and focus on speaking clearly, which is one the things that your pronunciation score is based on. One of the techniques is pronounce the vowels clearly and accurately.
Here are some videos about how to pronounce vowels from BBC learning English.