You've planted annuals. Annuals are but one season IF you are religiously deadheading them. Then they will grow until the first frost. Annuals have but one goal in this life and that is to produce seed/babies. As soon as they produce seed from their flowers the entire plant starts to die. The plant has done its job, it has produced seed.
Deadheading takes some guts to do at first. I was responsible for HOA entry 'color spots'...I went in and took off almost every single flower. Lots of passerby people and homeowners stopped and asked in horror what I was doing. I told them to check back next week at the same time.
And I had some amazed crowds. Flowers so profuse that they couldn't speak. Cut the flowers off so that plant is unable to make seed and that thwarted plant will grow vigorously producing a larger plant and far more flowers. Within a week. Deadheading as much as possible but there are so many flowers it would be impossible to dead head perfectly. The first couple of times it is important to be brutal but then your plants will out flower your pruner's capabilities.
I would help you to use a combination of perennials and annuals to be more impressive. What you've done is amazing for color.
If I were you, I would double the depth of that bed, even triple it. You need a constant backdrop for your flowers such as Pencil or Sky Ilex. Vertical shrubs, little columns. Softened with a 2' or less mass of shrub such as Hebe 'Quick Silver' Then plant chunks of Crocosmia, yellow or reds, stay with one color, intersperse with Woolly Lambs Ear, (cut the flowers off immediately from the woolly lambs ear).
I would get rid of the 2X4 edger...to triple the size of this bed. My suggestions are just suggestions I came up with off the top of my noggin. I have a video on how to do the best edge between a lawn and plant beds. I use no other type of edging if you are interested I'll give a little video.