The initial thing that comes to my mind is the thought that loosing excess weight could be of great benefit and that notion comes into my mind because I started Taekwondo at 11 (57 yrs now; or 46 years training and teaching -- before there was an ITF or WTF incidentally and on to today...) and I now fondly recall the struggle I had until loosing what I like to imagine was baby fat but was really hard fat and short ligaments and thick bellied muscles. Twisting exercises are and were helpful so long as I kept them quick and slimming as opposed to those that actually built the transverse abdominal muscles (as in punching drills) windmills while on the floor (saves one from potentially herniating a disk while standing windmills can herniate disks of the lower back-sacrum and ones where you reach through and then stretch backwards and then side to side w/ full motion through out -- not hurried through initially (always best technique) -- then increase speed and time or amount of drilling. Massage, and heat; acupressure etc.
Diet, of less meat where too much meat is the problem with most Western diets -- I read where a portion of meat should be about the size of a deck of cards. We synthesize our own protein and our muscles from that.
I heard a question asked of Biology students, where does the material for a desk come from? And, the graduate students (at MIT) where asked to graphically represent their beliefs -- nearly all or all showed the root system as being the source whole in actuality it is the leaves and sunlight -- photosynthesis that created the mass for lumber and not what was glossed over in the students understanding.
But, don't harm yourself in stressing and straining -- it will happen over time or you'll learn you were more built naturally to fly -- some folks are just built more inclined to flying and not flexibility per se. Not one thing in martial arts comes quickly and if it does is quickly eclipsed by a better understanding of what is set out on. You are speaking of an exercise, side rising kick which has very rare application -- disarming an opponent of a knife or blocking a punch, popping an opponent under the chin and else wise warm-ups -- don't be boggled down by wanting to excel at the perfection of a single exercise when more and other exercises, as you already are aware will bring you to your goal of mastery as -- you have the attitude and understanding already.
I like rice -- we eat too much muscle thinking we will gain muscle by eating muscle -- no. Exercise requires energy, and light digestible stuff. My Grandmasters were really big on soups on soups in summertime and Believe it or not that subject of soup, and benefits of soup constituted quite a bit of time and discussion by my teachers. Not until I got older and developed more refined tastes did I learn to appreciate a good cold )but spicy) noodle salad and hot soup on a summer day.
Best to your travails and be cautious a herniated disk is not progress.