Assalamualikum! I should not be saying this but it is bothering me. When something bad happens to me I lose my temper and say very very bad things about Allah 
like he is not merciful, he is cruel, and I also curse Allah (Naudhbillah). Do I become a Kafir on saying this? Are all my good deeds wasted by saying this? If I don’t become a kafir, can I repent for these sayings?
Answer:
In the name of Allah, the most Beneficent, the most Merciful.
Any person who associates a quality to Allah which is not befitting for him becomes a disbeliever. For instance, a person says (may Allah Forbid) Allah oppresses etc, then such a person becomes a disbeliever. (Umdatul fiqh p62 v1)
Any person who denies a quality of Allah Subhanahu Wa Ta’ala also enters the frame of disbelief. For instance, a person says (Allah forbid) Allah is not the most merciful) then such a person becomes a disbeliever. (ibid)
With regards to your question, your words have left you with severe consequences. It is compulsory that you renew your faith, and make sincere tawbah (repentance) to Allah Subhanahu Wa Ta’ala for what you have done. (Aap Ke Masaail p70 v1)
Only Allah Knows Best
Written by Molana Ishaaq Hussain
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham