When I was learning my first language (French) , there was always the fear that if I started speaking French with a native that I would take an eternity to finally say something, all while butchering their language. This meant that I became obsessed with wanting to say things perfectly. While this does have it's benefits it did give me a serious case of tunnel vision when talking and also really served to exacerbate my 'long pause' syndrome, as I became completely incapable of saying anything outside of the one true perfect sentence. As the years went by, and I finally forced myself to speak more and develop new ways of thinking to finally achieve fluency. I realised that I can't know all possible words and there will always be some things which elude me or things which I forget. The trick is the way the learner handles those situations.
Filler words: When you're really stuck and you need some time to think I always find it useful to have some words that you can use in any situation to pause for more time. This can be sentences like 'let me see', or 'let me just think about that', or even things like 'hmm that's an interesting/difficult question'. These handy little phrases fill up that otherwise empty awkward silence and remove some of the stress. It can be very easy to completely freeze but if you can say something it can release some tension and allow you to get your thoughts together.
Replace a word with a sentence: This is maybe a little more advanced but beginners can also use this in certain situations. Sometimes I really just can't remember a word and I have to think of another way to describe what I mean. (If young me were in this situation he would just have a complete nervous and mental breakdown and the conversation would come to a dead stop.) Let's say I've forgotten the word for 'cup'. Instead of staring blankly at my conversation partner while I ruffle through my memory banks I could just say something like; 'the thing we use to drink tea'. At this point the person you are talking to will prompt you with the correct word and the conversation can continue.
For beginners please don't be shy to use mimes or to make some silly noises to get your point across. At the beginning it's important to get your point across and to pursue the conversation.
Don't make it too complicated!: This is very important and I wish I did this earlier. When I first started speaking I always made my life too hard by trying to include various different tenses and relative clauses etc. Usually when people speak, they use simple sentences and the structure can sometimes be split up or maybe even moved a little (depending on the language). At the beginning don't get bogged down in the intricacies of the languages too much, keep your sentences succinct and simple. Try to pay attention to how other people talk and notice their speech patterns, usually they are quite short.
Make the mistake: There are of course times when you might not be able to think of another thing to say, and you know that you will definitely make a mistake, so sometimes you just have to take the plunge and make the mistake. When I start out with a language I don't know the conjugations of all the verbs in all the different tenses, and so I know that I will definitely get something wrong but I have to plough ahead. And if you do you will notice that the person you are talking to will usually help you out (if they see you're struggling) or ignore the mistake completely so as to not ruin the flow of the conversation. Most of the time there is no reason to be worried and people are generally pretty helpful and patient. Also, making a mistake can lead to some really amusing conversations by itself!
Don't let these mental barriers stop you from talking to people and really engaging with the language, Please remember that most people are very kind and happy to speak to you in their native language. They usually appreciate someone going out of their way to learn about their culture and will give you all the help you need. Just remember that you're not alone, and many other people have these mental barriers and you can get over them. You just need to take the first step.