Do you you know that the correct sound is extremely important? As a portuguese teacher I see that so many people struggle when they have to pronounce them.
According to the the language you are use to this might not be a big deal, however in portuguese when you pronounce an open vowel instead of a closed one you take the risk that you totally change the meaning of all the sentence.
I think the best approach to solve these little challenge is to have fun and playing the role of a native speaker. Listen to the teacher and just imit him!
Mimic not only the sounds but also the body movements and please don't take yourself so seriously!
When we learn something new we are like little children, so let's have fun! :)