Usually SSRI's decrease premature ejaculation
fact it can make it downright difficult for
some to ejaculate at all.
Most 20 year old males get exited quite easily, so don't worry about that. it will most likley decrease with age.
As one who suffers from General Anxiety Disorder and severe panic attacks (and depression) from my expe
riences, you have to learn to try not to worry about everything under sun. Get checked by a Doctor if you need to, but most likely everything is just fine.