Before you jump ship, make sure you are really experiencing this side effect. I think too many guys have one instance where they can't get hard on the drop of a coin and start freaking out that they are having side effects, which leads to excessive worry. If you are trying to get it up to have sex, then thats another added pressure worrying you.
Before I ever went on finasteride, I went through phases where I couldn't always get it up, even for sex. It was always due to stress, either general or in-the-moment pressure.
If you can't get it up when you are completely relaxed, with no drugs or alcohol in your system, then there might be a problem. Morning wood is a good guage as well, but not every man gets that.. I have it maybe a few times a week, and its usually only when I get to sleep in (as opposed to being up at 6:00 for work).
Maybe some encouragement, i've been on finasteride for over 2 months without any ED problems or other sides to speak of.. Except for a testicle ache that lasted a few days.