In the first bouquet, the initial work looks good but the arrangement disassembles itself over the course of a few minutes. The stems begin to slump, the flowers get pushed down too far, and the flowers begin to disperse. This is because the shape of the arrangement below the flowers was not defined. Floral design is not placing pretty flowers in a vase as much as building the framework that will support the flowers. Spend a few moments thinking about how the stems will cross, where the weight will sit, and where the heights will lie, and it is not as difficult to arrange flowers as it seems.
One good exercise to do with flowers is to begin with a simple container and just five flowers. Select flowers of different heights and weights. Place the tallest flower off center, then put two flowers in, that cross over the rim of the vase. Stems that cross make their own tension. Then fill in with the last two flowers, in between the first flowers, making sure that the side profile of the arrangement is triangular. This even with such a small bouquet, introduces the concept of balance.
One thing I see that I would correct is that all the stems have been cut to roughly the same length. When flowers are all the same height, the arrangement flattens out into a half dome. The flowers need varying heights. Take the time to cut a little more off each stem, each time looking from a different angle. Let one flower come up just a little above the others, and place another a little lower to make the eye flow in the arrangement. The movement keeps the flowers from looking so stiff.
Practicing just a few minutes a day will teach your hands where to place the flowers for balance and proportion. Spend a few minutes placing three flowers in a container, then stand back to look at the lines. Take one flower out and replace it. It is not so much adding flowers to the bouquet that teaches as much as taking a flower out and replacing it. Take the next few minutes rotating the bouquet as you take a little off of the height here and there. Training your eye to notice when an arrangement is out of balance before it is very bad will help your designs in the long run.
If you are getting frustrated, it is because you are trying to fill in every space in the arrangement right now. Negative space is not something to be feared. Leave holes in the arrangement for each flower to fill in. Watching how the space between the flowers affects the arrangement will make a big difference. With practice, your hands will begin to understand the tension, balance, and proportion in flowers and the arrangement will begin to hold itself together instead of falling apart.