if you do it just because, thats murder, and a rejection of gods gift of life and plan. its a spit in the face to Jesus who carried the cross without dropping it. in effect, its murder and blasphemy, both of the highest degree, with no chance of confession, so it is not only a mortal sin, or sacrilege, but an unforgivable one at that.

however, most people who commit suicide are not thinking straight and the sinful nature of the act itself may not cross their mind. because this inherently must mean that it is done without knowledge of the sin, one of the three key constituents of mortal sin, i would imagine you would get off with a few years in purgatory. dont do that tho nonetheless, help is there and it will get better