If you struggle to put your words into sentences, that is probably because you don't understand the structure (the grammar). You cannot learn every single sentence you're going to produce by heart! I suspect that you are forgetting your vocabulary in your sentences because your brain is too occupied at working on the structure of the sentence, and you simply cannot be doing both at the same time. At some stage - and it looks like it's now - you're going to need to work on your grammar so that you no longer need to think about it so much.
I would suggest that you start with verb conjugations. Verbs are the backbone of the French language; without them, you cannot communicate.
Make a list of the tenses you have learnt so far. Learn the basic rules for conjugating your regular verbs in all these tenses.
Then - and this is the really important bit - take the common irregular verbs (I'll list them below) and learn your tenses for these. The trouble with irregular verbs is that they are the ones that you are the most likely to use! The most common verbs are irregular because over many generations, their frequent use has gradually deformed them: wear and tear, you could call it.
As you learn your verbs, make up short sentences using them - in a wide range of tenses - in combination with the vocabulary you are learning. This will help you reinforce both your verbs and your vocab.
Here are the essential verbs. In brackets I have put verbs that are conjugated in the same way and that you may want to use).
faire (also défaire, to undo; refaire, to redo)
voir (also revoir, to see again)
venir (also revenir, to return or come back; devenir, to become; se souvenir, to remember; tenir, to hold)
prendre (also apprendre, to learn; comprendre, to understand)
dire (also redire, to say again or to repeat. Interdire, to forbid, is almost the same but not quite, to be careful)
mettre (also permettre, to allow; promettre, to promise; admettre, to admit)
connaître (also reconnaître, to recognise; paraître, to seem)
recevoir (also apercevoir, to notice)
ouvrir (also couvrir, to cover; découvrir, to discover; souffrir, to suffer; offrir, to offer)
There are obviously more, but these are good ones to start with.