I would be very grateful for any advice, especially if someone has similar experience. This problem happens sometimes, not always. However, it has remained unsolved for few years, and is one of the reasons that made me switch from breath to other meditation objects.

Namely, when I focus on the breath, my body stops breathing. Here's how the story sounds like: 

1. First, I say to myself "I won't control my breath, I will just relax and let breathing happen naturally".
2. About 30 seconds passes and there is no inhale. It seems like the body "forgot" how to breathe spontaneously, and breathing apparatus won't move until I control it. Then, of course, the feeling of suffocation starts, and subtle anxiety along with it. 
3. As unpleasantness grows stronger, an inhale happens, but I can't clearly perceive whether it was spontaneous or controlled. That triggers further feelings of guilt, doubt and anxiety. This creates negative feedback loop that intensifies with every breath and creates this nasty anxious "fight or flight" tense state.
4. This all causes overthinking about how to solve this, during and after the meditation, and triggers negative self-thoughts ("There must be something deeply wrong with me").

I feel like the cause of this is some tension / blockage in the solar plexus area, connected with the need for control. It may be some kind of unconscious fear to let natural processes happen spontaneously - maybe the unconscious mind has a belief: "if I am aware of something, I must control it, because I am scared to let things happen on their own." I speculate that there is some psychological "thorn" (shame and anxiety) deeply “stabbed” in my solar plexus, blocking the free flow of life energy.

My overhinking mind has produced gozillion theories about how to solve this, so I can't choose which one to follow and don't have a clear strategy (analysis paralysis). Here are some ideas:

- Control the breath, but imitate the rhythm of natural breath and notice that feeling (illusion) of control is also occurring in awareness.
- Note feelings and thoughts associated with unpleasantness (very hard to do, since thoughts are fleeting and I can't pinpoint where the feelings are. It doesn’t seem like noting make me more aware of anything, it feels like I am just saying words).
- Strong determination sitting: just endure all the pain and confusion until mind realizes that it won't suffocate.
- Focus on the totality of unpleasantness, and notice the characteristic of no-self (notice that you are watching it, therefore you can't be it).
- Make one part of the breath controlled, and other spontaneous (for example, exhale fully and let inhale come spontaneously).
- Taoist-like techniques where you focus on the area of tension in the body and relax it.
- Reichian exercises or other specific breath exercices.
- Choose another object of meditation - develop your concentration and mindfulness skills with it, and then apply those skills to clearly see the unpleasantness associated with breath.
Etc. etc.

I have a similar problem with controlling the breath, which is holding back my progress. It started when I was a teenager, trying to meditate using books.

For me, when I focus awareness on the breath, I think I am controlling it. I don't usually feel choking sensations, unless I try and release the false sense of control. What I have been trying, with limited success are:

Put attention at the nose, briefly move attention to the abdomen and intend to cause a breath, move attention quickly back to the nose. By doing this, the observation, and intent to control the breath are happening at different parts of the body.

Deeply inhale, then put attention at the nose and do not try and control the breath. The body will naturally force out the breath. This works well when lying down, since gravity will assist the exhale.

Put my attention into some activity, like reading. Keep the sense of the breath in the background of awareness. Some times this creates a sense that the breath is happening separately from my object of attention.

Thank you for your advice!

I also want to share a method and framework that I invented in the meantime. Here are the steps:

1. Preparation: pranayama – inhale for 4 seconds, exhale for 8 (in order to make the body adapted to lower oxygen levels, so the feeling of suffocation is less intense later),

2. Exhale by consciously contracting belly muscles, inhale by letting them go (feel the spontaneity of inhaling this way, relax into the pleasantness of letting go; it’s like doing progressive muscle relaxation with your belly),

3. Stop controlling the breath in any way. In my case, after an exhale there will be no spontaneous inhale. Have this notion: it is not that the body is “lazy” to inhale, it’s not about lack of activity; on the contrary, the reason there is no inhale is that there is this subtle tension in stomach and chest that keeps it from coming. So, there is no effort needed for spontaneous breathing, but unconscious tension is blocking it. It is the same natural tension that is needed for the end of an exhale to happen, the problem is just that this tension continues instead of transforming into inhale.

4. Become aware of this subtle tension. As you pour your awareness into it, and as it stops being unconscious/automatic, at some point you will get the feeling that the tension is not autonomous – you will feel like it’s there because “you” are maintaining it. Then, you will get the freedom of choice to let it go. The focus is passive (“I let this tension to come to me / show up”) and not active (“I will search/find/discover the tension”). It is important to have the attitude: “I will be aware of tensions, accepting them, and letting them dissolve on their own, maybe with gentle help”, not the attitude “I am focusing in order to get rid of those tensions by relaxing them”. Also, don’t think about spontaneous breath as a future goal in mind, your only goal is to focus on tensions in the present moment and gently allow them to relax.

5. In time, with many repetitions, the breathing is going to become automatic – it is going to feel shallow but pleasant, spontaneous and intimate. The body will feel relaxed and at ease. From this place you can do samatha practice or any other meditation you prefer. Maybe there will remain some illusory feeling of guilt (“Did I control this breath? Did I made this inhale or did it happen spontaneously?”). Those doubts are irrational and false, just ignore them.

6. In some cases, this spontaneous breathing won’t happen – instead, the feeling of suffocation is going to emerge and it may destabilize your concentration. In that case, don’t worry. The fact that you were aware of the tensions for some time is going to make it easier to dissolve them in the future. Just start again at step one (pranayama).

This method helped me, but of course, variations may be needed for different cases.