The first one is that I have trouble letting go of my breath, letting it flow naturally and just observing it without judgement.
It seems like every time I focus on the expansion and contraction of my abdomen, I tend to force the breath or at least try to exert some kind of control over it as if I'm subconsciously resisting relaxation for fear of doing something wrong.
This one might be pooh pooh'd by others, but maybe just try noticing that you are breathing, rather than getting caught up focusing on any particular area. There's all sorts of school of thought that say focus on the nostrils, or focus on the abdomen, etc, but to me that ends up creating a blur between breath awareness and body awareness.
The second issue is with applying all of this meditation (I'm up to doing 30 minutes a day from 20 minutes) in my daily life.
I constantly forget to be mindful and present to the moment in my daily routine and very often get stuck into the same negative thought patterns that made me depressed years ago.
This is a question and concept I have struggled with in the past many times, how to transfer the experience on the cushion to everyday life. What I have found is you need to be able to stop. Stop thinking, stop measuring, stop comparing, stop judging, etc etc.
While sitting, you should be able to see more and more the arbitrary, subjective nature of your thoughts and beliefs - there is reality, and then there is your spin on reality. As your practice sees this more and more, you should be able to have that spill over into your day to day life. Have a look at your habits, your attitudes, and perspectives as you go about your life, see them for what they are, and stop them. Where you might get irritated for instance, notice that arising, and realise that is an automated response, just like any other reflex. You may not feel brilliant, but being aware of what your mind is doing is winning part of the battle. Over time as you take this approach with all your perspectives that incline you to depression, you should remove their hold and power over you.
I have found with depression or ill feelings, there's an amount of chicken and egg with them, ill feelings will lead to ill thoughts, and vice versa. If you can gradually (and note gradually, it is not going to happen instantly and may take weeks or months) relax both your body and your mind in tandem, you should get yourself closer to peace.
I have also learned that a large amount of this is beyond words, and hard to enunciate. Perhaps also useful is to stop talking to yourself either in sitting or in your day to day, removing the language in my thoughts has led to greater simplicity and less rumination.
You should hopefully be seeing some sort of gradual progression, it is going to have peaks and troughs though, just keep at it.