New Rider: Need a little guidance please while riding at a trot?

[calley112]

i am a new rider, overweight (working on it) and i am having a heck of a time with i my balance. i feel that when the horse goes even at a fast walk i am going to pop right out of the saddle! i have had only 4 lessons and my instructor tells me this is normal and i will get it .... but i am loosing my confidence. in the 4 lessons i have ridden 3 very different horses (thank God they are patient) but i am looking for advice please on how to actually stay in the seat comfortably and secondly how to not bounce around like a fool while everyone else seems to be doing just fine with their little up and down movements. - oh yeah, my instructor has told me to keep my seat at a trot - on only one horse can i do this - the other two i look like a drunken sailor. Please only answers where I can achieve something from. thanks so much everyone and happy new year!