Separating connected arbitrary Objects

Hi everyone, I am very new in image processing and working on segmentation.

I am using Sobel for segmenting the image as described here:

https://mathworks.com/help/images/examples/detecting-a-cell-using-image-segmentation.html

However, if I use the threshold calculated by Matlab, sometimes weak edges can't be seen, and there is connected objects as a result:

There should be different objects (blue ones detected as parts of 1, green ones detected as parts of 2). If I increase the threshold (which I don't want as in reality, I can't change the threshold manually), blue ones are separated but I lose some small objects.

Can anyone help me stick with the default threshold but still can separate the other objects. I tried with "watershed" but it overseparated my objects, for example object 1 (very thin and wide one) will be separated into many smaller objects.

Thank you.