This is extremely common Tatjana, just do not worry about it. If you cannot remember the whole thing, make up the ending yourself. Over time, you will get a lot better at remembering long sentences, but really, honestly, it is no big deal and not worth getting frustrated about 
I suggest that you push on. I listened to Challenges 12 - 20 during my drive to and from Abertawe today, and you will get lots of practise with the key stuff in the upcoming lessons!
Most of all, take a deep breath, laugh at your mistakes, enjoy your Welsh and try not to be hard on yourself. Everyone has gone through what you are experiencing, and come out the other side a Welsh speaker - and so will you. There will be down days, but over time, the highs will overwhelm the lows and as you grow in experience, you will be happy to go with the flow, believe me. The best of luck with Challenge 14, and post back with how you get on; I am certain that your story will be here to inspire other learners to persevere long into the future!
Hwyl,
Stu