> What can I do in those cases to gain insights as to where my suffering is coming from?
Definitely signup for a course with Bhante. You can get a number of insights during that. It can be done from home and is free of charge.
> Without understanding the roots/causes of my sadness
Roots/cause of sadness (or any other unwholesome mind state) is really very simple: the mind takes delight in clinging to them. At the same time the mind does not see the dangers (dukkha) associated in clinging to those states.
Why does the mind do that? That's is because it hasn't looked closely into the above process and therefore has poor judgement about what makes it peaceful. This poor judgement is also known as delusion.
Mindfulness, when done right, sheds light into the above process. Once the mind internalizes that the delight is just not worth the dangers, it automatically becomes dispassionate to those states. Only then it is freed of sadness etc.
Note that you cannot "will" the mind into dispassion - it is out of control. But you can certainly use the properties of the mind itself to creates conditions that will eventually lead to dispassion. E.g. a strong mindfulness practice.
> Sometimes I feel sad but I don't know why
Following is not a substitute for a formal course with bhante. Next time you get into the sadness loop, I invite you to try the following
- Observe when sadness arises, what arises with it (sensations, thoughts, perceptions)
- Observe when sadness leaves, what leaves with it.
- What in the above were you delighting in i.e. liking (e.g. could be the perception "I deserve better")
- At the same time recall what negative effects that came about between steps 1 and 2 (e.g. perhaps ignored someone important, or wasted valuable time)
- Ask yourself - what you liked in step 3 worth all that in step 4? If not do you want to really continue the process in the future?