Hey,
Your Lithuanian is pretty good! Therefore I'm gonna add some corrections which are more related to the meaning of your senteces, rather than the specific grammar. 

Please don't get demotivated when reading these corrections - I mean all best. Your grammar is really good, and I believe I understand the message you're conveying. Which is exactly why I wanted to share corrections that are aimed at deeper and more complex understanding of the language. It's not an easy one to crack, but you are definately on the right track! Keep it up!


1) Man truputi keista. Kodėl?
Keista (pronounced with stress on i) is more like a description rathe than feeling. Therefore, "Kas?" is a little more fitting than "Kodėl?"

2) Aš labai bloga
This actually means that you believe you are a bad person, and I hope that is not the case :-)
I suppose you meant to say that you believe your skill in language is not good, which would be better expressed like:
Kai mokausi kalbos, pastoviai galvoju, jog man nesiseka ir kad nemoku gerai kalbėti.
If what you meant was "bad learner":
Kai mokausi kalbos, pastoviai galvoju, jog man nesiseka mokytis ir kad nemoku gerai kalbėti.

3) Turiu problemų

4) Šiandien pažiūrėjau filmą.
This would transalte as "today I have looked at a movie". It is completely understandable, but would be more "Lithuanian" to say:
"Šiandien žiūrėjau filmą" or "Šiandien mačiau filmą".

5) "kai klausiasi" actually means "when asking about something" :-)
I think you meant "klausosi" - listening.

6) "Jau nežinau, kaip aš žinau lietuvių kalbą". 
Again, this makes perfect sence in your context, but if taken out of it this would translate as "I have no idea how did it happen that I know Lithuanian". This particular case is very similar to English way of saying this sentence: "I don't know anymore how I know Lithuanian language". The sentence is correct, but if intention was to say "how well" instead of "how come", the word "well" must be added.

So for this sentence, it would be more correct to say "Jau nežinau, kaip gerai aš žinau (moku) lietuvių kalbą". 

7) "Negaliu tikėti"
Putting it this way is again perfectly understandable, but it would be more correct to say "netikiu".
"Negaliu tikėti" means that you cannot because you're not allowed or lack ability to, whereas "netikiu" literally means "i don't believe".
If you would like to express it a little "softer", you could say "nemanau". Believe is a strong word, whereas "manau" is more like "i wouldn't think this was possible, but what if i'm wrong?".

8) "ne galima"
In this case it has to be "negalima", as in - one word. It's something that even some Lithuanians struggle with.
The rule says something like: if you mean to say the opposite of the original word meaning, the "ne" is connected to the word. So: galima -> negalima (opposites), eiti -> neiti.
But if you want to compare something (I use not blue, but red pen), "ne" must be separate. You can find more details in here: http://lietuviu7-8.mkp.emokykla.lt/lt/mo/zinynas/dalelyciu_rasyba/ (it's in Lithuanian, but I belive your understanding of Lithuanian is at the level where this should make sense in Lithuanian, or when translated with Google Translate).

Keista...

Man truputį keista. Kodėl? Visada kai mokausi kalbos manau, kad aš labai bloga ir nemoku kalbėti gerai. Visur darau klaidas ir turiu problemų. Žinau, visi turi. Bet... Šiandien pažiūrėjau filmą lietuviškai. Tai buvo pasaka skirta vaikams, nebuvo labai sunku, bet visada sunkiau kai žmogus turi suprasti be teksto, tik kai klausosi. Supratau beveik viską, tik nelabai vieną dainą. Ir todėl keista. Jau nežinau, ar aš moku lietuvių kalbą? Visa laiką sau saukiau kad nemoku ir nemoku. Gal truputį moku?
Tik negaliu patikėti, kad žmogus pats gali išmokti kalbos. Man visada visi sakė kad taip negalima. Taip, neturiu mokytojo. Niekas mane nemoko.
Dabar jau nieko nesuprantu...

(There still are a few mistakes. Vis dar yra keletas klaidų.)