Yes. I can work out in my head exactly what I want to say, and it sounds good to me.
Then........I open my mouth and I go into panic mode and it all turns to rubbish.
What a relief - I was beginning to think it was only me! I have all kinds of conversation scenarios in my head, but as soon as someone else speaks they disappear! Even the simplest thing is beyond me, sometimes - it is so frustrating, when you know what you should be saying, and it just won't come out of your mouth. We have several, very helpful, Greek friends who keep saying "you must speak Greek", but the minute they start answering our simple, polite enquiries we are lost. At least they are all willing to help us, and we have a lot of encouragement, rather than people taking the mick, as happens all too often to non-English speakers in the UK.
Just keep on thinking of all that complicated grammar, and I'm sure it will be fine in the end - if we don't end up in the nut house first!