The sentences are pretty good. I made some minor revisions below:
1. I am beginning to be afraid of myself OR I am beginning to be afraid myself. These sentences have 2 different meanings. The first says that you are worried about some action that YOU have been doing. You are worried (afraid) of your own behavior. In the second sentence, the word "myself" is used to emphasize that you are afraid, but the cause of your fear is outside of yourself.
2. I can only speak some of these languages fluently. Adverbs ("only") can be difficult because they can move around n the sentence. In this sentence, it is correct to move it towards the beginning of the sentence so that it refers to everything that comes after it ( "speak some of these languages fluently")
3. The sentence is good as it is. The word "popped" might be British English. In American English we say "blew".
4. Don't make impulsive decisions OR else you'll regret it. The phrase "or else" is very common.