Pam, 

Maybe you are trying to construct a normal, logical argument like you would in your mother tongue but your language level can only allow childish sentences. Many adults can't speak because of that.

They are subconsiously trying to form sentences and have conversations the way they do in their own language and their brain freezes because there is no vocabulary to support the speech.

Speaking Chinese in the UK was really uncomfortable for me. I couldn't speak in the shops, and I couldn't even speak to my friends who were native speakers. 

However, when I went to China, it was absolutely necessary... I had no other choice, and in this way it forced me to speak the language. 

This really broke the ice, and when I returned to the UK, I no longer felt uncomfortable. 

So my advice, which is generally applicable for any aspect of language learning, is to absorb yourself in the target language. Perhaps take a trip somewhere where the level of English is extremely low. 

You'd be surprised, people are generally tolerant even if your speaking is nervous and unclear - after all, you're making an effort to learn their language, and often they don't know yours. :)

Maybe you should try speaking with an english speaker who is also learning spanish. It can ease the tension of forgetting words and be unable to comunicate with your partner. If you don't know how to say something in spanish you can always say it in english and the other person will understand, and maybe tell you the word in spanish. Once you get some confidence speaking the language, you can try talking with an spanish native speaker. :)

P.S: I'm going through the same with learning english.

I like that one about the alcohol.  :-)

This is very common. When we are put on the spot we freeze. 

You may find it helpful to practice stock phrases, saying them over and over, to get the conversation started. Be able to just recite stuff about what your name is, where you live, etc. When they respond, you may have to just say "yes" or "no" to things for a while.

Another thing that helps is to learn how to say, "How does one say?" Practice saying that so it rolls off your tongue easily. That way, if you are searching for a word, you can actually ask them. I did that in France when I wanted to say that a spider was in my bathroom. I said, "How does one say that little creature with eight legs? Yes! That was in my bathtub!"

Finally, the thing that helps me unfreeze is alcohol. Not so much that you're slurring your words but it's a fact that alcohol lowers inhibitions and that inhibitions are what hold us back from speaking. Obviously, this may not always be practical. And for some people it may be inadvisable. But there is a foreign language club that meets at my local pub and I hope to go there to speak Spanish at some point soon. I'm sure that they hold it in a pub for a reason!