i have been teaching swimming lessons to people of all ages - i have 10 years of experience between group and private lessons as well as teaching lifeguarding, triathlon and stroke improvement classes. i now own and operate a private swim school. what everyone has been saying about being tense is 100% correct. swimming is far more about learning new motor patterns and being comfortable in the water than it is overall fitness. obviously, fitness may be a limiting factor but if you are an adult learner i promise you technique is your number 1 barrier. my observations on how to correct front crawl...
to improve bilateral breathing practice side glides on each side. the arm closest to the bottom should be fully outstretched, ear down to the shoulder. the other arm should be down at your side. ensure your ear being held to the shoulder stays in the pool and your head is neutral; do not force or tilt your head above the water. this will compromise your body position. instead, roll backwards slightly. a cue that helps is letting yourself sink - this will ensure your body position is optimal in the water and forces you to roll slightly backwards to breathe as opposed to tilting your head up on an angle. keeping your head in a neutral position will ensure your legs stay balanced near the surface. then, kick like hell. with practice, you should be able to do 10m side glide on either side - this is sufficient for bilateral breathing purposes.
incorporate this into a drill with a kickboard. front glide (face in, both hands on the flutterboard) to side glide. do not move your arms constantly as if you were doing front crawl. instead, focus on bringing one arm down to your side as you roll sideways to breathe. ensure you breathe to opposite sides. you should have a full exhalation as you do your front glide before you roll out to your side. do not exhale when you come up to breathe, only inhale - you should have exhaled fully underwater. ensure you are rolling your body from the hips and not lifting your head. tuck your chin in farther during your front glide if you are having trouble keeping your ear in the water when you breathe; this ensures your head and neck will be in a neutral position when you roll. ensure you have a strong overarm recovery as you return to the front glide position and do not look forward. take away the kickboard when you are able to complete this for several laps. make sure you don't spend a lot of time breathing. side glides are hard as hell and your body wants to be on its front. the longer you spend breathing, the more tired you will be.
as far as comfort goes, practice front, back, and rollover floats and glides. do glides with and without kicking. practice letting yourself sink to the bottom (exhale) and float up again. attempt to transfer your weight - ie, do a handstand. the more directions you can move in the water, the stronger your breath control will be. the reason kids are kicking your ass all over the pool is because they play in it all day long - they know how to catch their breath, how to roll over, how to make it to the side when they are tired, etc. spend some time fucking around, diving for rings and whatnot. practice treading, jumping in and treading, jumping in and sinking to the bottom and pushing yourself to the surface... etc. multidirectional movement is key. front crawl is a very, very limited stroke and is not practical for leisure swimming which is the #1 way you will become comfortable in the water.