Ok - first to address from a breathwork standpoint - try focusing on initiating the inhale from the lower rear of the diaphragm and continuing that intent throughout the inhale - another way I've mentioned is imagine the spine is a train track and you have a toy train, drive it down the track on inhale, back it up on exhale. (not as low as genitals, not as high as heart.) But an extra point of focus there, relax on the exhale. What that does is establish a smooth motion in the diaphragm that lets the diaphragm's motion move smoothly past the openings - so two major heart related ones are right there, the aorta and vena cava. With that motion continued, eventually that makes the heart light, buoyant, effervescent even. It can sit and shine comfortably, not feel heavy at all.
So with that physical bit down, here's another thing you can try to help open the area - a friend of mine related this in a sort of de-programming context, but I interpret it a little differently in a more positive manner. Think of the heart healing sound, he (huh? yeah, that phonetic
) and without making a sound, imagine resonating that as loud and as deep as you can in your vocal range - at the heart. Then do it again, but in the highest vocal register, similarly resonated at the heart center. My wife's name was a good one for me to use there also
But anyway the deprogramming technique was using the word no - while I found that effective, I simply choose to focus on more positive forms. (As with any practice messing with energy potentials, always stillness afterward...you probably know that but I just continue writing it always anyway because it is important for anyone to relate to their own practice.)
One other practice that would do some good is called pulling down the heavens - you can use the heart sound for that also - raise the hands on the inhale palms up, to a comfortable height, then turn palms downward and exhale with the heart sound while 'pulling down the heavens' - so a nice relaxation from the heart center coinciding with the sound - it can of course be done vocally or subvocally, they are different levels of subtlety so if you do the healing sounds, I recommend both ways. If you dig on my 3 dantiens work at all, this also relates to hexagram 12 - so a gentle waxing of the 3 dantiens on the inhale and waning on the exhale.
You can also do some more laogung breathing
Many methods, hehe
Even in mildly complex systems, any outcome is the wrong thing to target, with the process being where the focus should be.