orgasms

I've reached orgasms when my partner stimulates but whenever I do it alone I can't reach one because my hands get too shakey as I reach a point of near climax. does anyone know what I can do to achieve an orgasm alone? it really gets too much because i can't see my partner for a while at a time due to uni. I'll see him next in Feb and I'm horny af like all the time. help!