One of my good mates wanted to jump higher but always over thought it so what I did was got her jumping a course at the height she was comfortable at (which was 70-80cm) but while she was doing the round I'd put a few of the jumps she'd already jumped up slightly and by the end of it she was jumping around 1m. Just find little ways to get your focus off the height of the jump
I was always nervous when the jumps got higher and I found that putting trust into my horse and telling myself to breath worked. Obviously take it slow and try maybe doing a one stride so that you don't have to find your distance as much. Whatever height you are jumping put as the first jump and the second one a bit higher and work your way up slowly, it may take a while. Enjoy it and tell yourself you are having FUN and before you know it, it will all come easier and easier by the jump! Another thing I learned is to never give up because I almost quit riding. I got so scared to trot a crossbar a year and a half ago and now i'm easily coursing 3'0"-3'6". The more strength you build in your legs and torso, the more you can hold yourself in a good position over jumps, and flatwork, flatwork, flatwork. It ALWAYS pays off trust me! I hope this helped!