Could you record it?
Ok, let's start saying I have "studied" phonetic for the last two or three months, until then I always focused on grammar, vocabulary, idioms, etc.; I knew that my pronunciation was bad, but people just made fun of me or they used to say something like "take it easy, I can understand you anyway" and nobody ever taught me or helped me to speak better, so I started exerting the pronunciation on my own with the support of videos on Yotube, etc.
I'm Italian and we have just 7 vowel sounds and some diphthongs; I was shocked when I found out that English has twelve or even more of them and I figured out where my accent comes from, I've always been speaking English with a half of its vowels sounds (I should also mention the th sound, but let's focus on vowels).
Anyway, I started to pronounce the vowel sounds singularly and the words in which they are present...ok I know that's always easy to pronounce a single sound than a word and a single word than a sentence....
The fact is, whereas I had some problems (and I still have) at recognizing (or should I say in recognizing?) and pronouncing the sound 3: (which is in words like word, earth, work, etc.) I easily recognized and I can pronounce individually the "schwa" sound (teacher, about, etc) , maybe because it's used in some Italian dialects or just because it's simple since it doesn't require the use of the tongue... anyway if I try to say "teacher", "about" or any other word that comprises the sound I pronounce it more like an "e"...
Is there someone else who has faced this problem? How did you cope with that?
Could you record it?
It isn't strictly necessary to be able to pronounce perfectly all of the phonemes of a given language (but it's good that you are making the effort to); if I were to learn Italian I would struggle to distinguish o /ɔ/ from ò /o/ and would most likely pronounce them the same; but you could still understand me. It would be fine, therefore, if you were to pronounce a full vowel in those place or better if you said a short "e" sound.
As for helping you, try exaggerating the schwa in these places; properly go for it and don't be afraid of sounding silly (but you may want to practise in private) and it might become more natural to you, to the point where you can do it normally in words. I don't know whether this would work, but it's worth giving a go. :)
One other thing, marcodraper. What may be persuading you to pronounce a full /e/ in words ending in '-er' is the dictionaries' addition of a bracketed /r/. This means NOT that 'you can add a hint of an /r/ and it'll never be wrong', but that there is an /r/ sound ONLY when there's a following vowel: /ðǝ ti:ʧǝr a:skt/ but /ðǝ ti:ʧǝ sed/.
In some dialects of English (the so-called 'rhotic' ones), there is always an /r/ sound, but it's not standard.