Well, you are learning a language that has at least 30 different spoken vowels, if you count long and short vowels separately and also stressed and unstressed vowels separately. (See this question). You have to count them this way, because for example the pronunciations of "Stall" (stable, barn) and "Stahl" (steel) differ only by the length of the vowel, and the two meanings of "umfahren" (1. drive around an obstacle without touching it, 2. make the obstacle fall down by driving over it) differ only by the stress that is put on the vowels.
Stall = [ʃtal]; Stahl = [ʃtaːl]
umfahren, meaning 1 = [ʊmˈfaːʁən]; meaning 2 = [ˈʊmfaːʁən]
30 is just the number of vowels that appear in Standard German words. But we use foreign words too, and often we also use the original pronunciation, so we also have for example [ɔ̃] in "Balkon" and "Beton" or [ɑ̃] in "Gourmand" and many people use [æ] for words like "Badminton" and you will hear it in Austria at the beginning of "erlauben". In German dialects you will find even more vowels that do not exist in Standard German.
I don't know any other language with a number of vowels as high as German.
So, you shouldn't worry about your troubles. Everybody, who learns German has these troubles too.
The problem is, that your brain is trained to recognize and reproduce only that sounds that you are exposed to very often. This is similar to recognizing fine nuances of colors. And the only way to come over these problems is training. Long and hard training. And this needs a lot of time, usually many years.
Here are a few suggestions on how you can improve your pronunciation:
- Listen to native German speakers. Watch movies in German language (use subtitles if you are not fluent enough to understand the dialogues) Listen to native speakers, even if you don't understand everything. Try to understand as much as possible. This will train your brain to distinguish between the different sounds.
- Talk with German native speakers as much as you can.
- If you are a member of a group of people who all want to learn german, try to talk with each other in German as much as possible. You can't train the new language if you always use your first language. (Although my wife and me are both German native speakers, living in Austria, where everybody is speaking German, we still often talk with each other in English just to train this language.)
- Don't give up. It will need some years, and the more you practice, the faster you will reach an acceptable level. I have heard Arab people speak German almost without any accent after just only 4 years.
Be careful with IPA-symbols!
Neither "[ü]" nor "[ö]" nor "[:]" are valid IPA-symbols!
The German letter "ü" can be pronounced as
- [yː] like in "müde": [ˈmyːdə]
- [y] like in "Büro": [byˈʁoː]
The German letter "ö" can be pronounced as
- [øː] like in "König": [ˈkøːnɪç], [ˈkøːnɪk]
- [ø] like in "Ökonomie": [ˌøkonoˈmiː]
- [œ] like in "löschen": [ˈlœʃn̩]
The marker for the length of a vowel is not a colon (":") which is either 2 round dots or 2 small squares, but a distinct signs that is made from two small triangles pointing to each other: ("ː")
A good description of IPA symbols is available in German language: Liste der IPA-Zeichen. There are descriptions in English too, but I didn't finde one that is as good as this German description.