Try to remember this one important rule when speaking: KEEP IT SIMPLE. Speak in sentences using grammar you have mastered. If shorter, more simple sentences are what you can form well, then these sentence structures are what you should use. If you try to speak in long, complex sentences that you still have difficulty forming, you will make mistakes and confuse your listener. Always stick to what you know.
Right about now you might be saying, "If I speak in shorter, more simple sentences, people will think I don't know English or that I'm stupid". This is simply not true! In general, when you are speaking, no one will notice that your sentences are shorter; they will only notice that your grammar is correct. They will think you speak very good English! They will notice, however, when you make mistakes in grammar, or when what you say confuses them.
Remember, listeners expect to hear people speaking with correct grammar, regardless of the length or complexity of the sentences being used. Listeners do not focus necessarily on the complexity of what you say, but the accuracy of what you say.
It is important to speak in sentences using grammar you feel comfortable with and that you have mastered. Practice increasingly longer, more complex sentences as often as you can. Ask for corrections when you make mistakes, and you will learn more quickly. The more you use complex sentences, the easier it will become.
Practice makes perfect!