It's a problem that every language learners faces: you're talking to someone in a language you haven't mastered yet, and all of the sudden you can't remember a word-or you realize that you haven't even learned it yet! The flow of conversation stops, your mind goes blank, your conversation partner searches your eyes for whatever you are trying to communicate, but all they see is panic. We all forget words(even in our mother tongues), but that doesn't need to shut us up. What will you do when this happens on you ?
The person you are spaking to should know the word you are missing just by thinking about the context (the topic and the sentence you have spoken).
If you cannot think of the word, you could ask the other person to help you.... just say "I'm sorry. I can't remember the right word. Can you help me?"
The other person may know the word (based on the topic/context) or he/she will ask you some questions which will help to identify the right word.
If I talk Chinese and I forget the word for chopsticks, I could say "The things you use to eat the food / The things that pick up the food". In otherwords, tell a little story or describe the thing to describe the missing word.
Forgetting a word happens to everyone.
Sometimes I forget English words and it takes me one minute to remember the word I wanted to use. And I'm a native English speaker.
A good way to solve this problem is by paraphrasing. You don't have to know all te words. In case you forget or don't really know the word you want, try to describe what you want to say. I'm pretty sure the person with whom you talking to will tell you the word after you have explained it.
As for being nervous while speaking, that's just a matter of practice. The more you do it, the easier it will get. Unless, you get nervous when you use your native language, too. Then, the problem would be something that doesn't concern language learning, but a personality trait.
I think there are several reasons for this problem. Firstly, you may stressed up during talking with some one you don't know in a new language. Secondly, you didn't improve the basic skills as well such as the vocabulary, the structure, the parts of speech and the pronunciation. Thirdly, you were not prepared as well in specific subjects before talking with your partner. Finally, you should study your self several times a day not less than an hour and imitate the native pronunciation.
The people I talk to are language partners who are trying to learn English. I don't let it bother me.
In fact, I almost enjoy it. It's a learning opportunity. I will stop and ask. I will try to explain what I mean in other words. My language partner helps. It is a good exercise in understanding and being understood.
I may have forgotten, or I may not know a word, but I am _functioning_ in my new language.
Several good things can happen. I may learn a new and different word that means something similar, for example.
And I've found that when we do figure out what the word is--I don't forget it. I really sticks in my mind because I <em>needed</em> it, and I was struggling for it.
I second what Marcos writes about paraphrasing. It is very practical and if you are lost for words, then you could even draw a little picture. When I moved to China the first time, I had to paraphrase a lot. For instance I always wanted to order 甜酒汤圆, but I just never the knew the name of the dish, so I always explained the waitress in Chinese: "It is white and sweet and like a soup, but it is actually not a soup, but a dessert and has rice in it." Ok, would be quicker to just remember the name of the dish, but anyway I normally got what I wanted.
Also, if the person you speak to, shares another language with you, you could always use a word in that language. If you are learning German and speak to a German and can't find a German word, just use the English one. If you are learning Japanese and speak to a Japanese, you could always write down the word in Chinese and chances are you Japanese conversation partner recognizes the characters.
And as you said, we all forget words, even in our mother tongues. Nobody expects you to be perfect.