Personally i am on finasteride 4 months now.My experience?Zero morning woods,zero erections,even when i fap my d1ck doesnt feel that hard and i am 25 years old.I also wake up at least 2 times during the night to pee since i started finasteride(which i am not sure to blame finasteride for cause i am drinking like 5litres of water during the day).Also i am working out and i lift and my gains are too slow(which i am not sure to blame finasteride for as well),plus some brain fog(which i am not sure to blame finasteride for).
I maintained my hairloss though and my hair look healthier day by day(which i blame finasteride for)
how often are you taking finasteride? i'd highly suggest coming down to every other day, or even just 3 days a week. give that a month or two and see if it helps. otherwise, finasteride may just not be for you (clearly, it's not for everyone, but you don't know unless you try). after 4 months of not being able to get it up, i'd say it's time to make a change. and if limiting your finasteride intake doesn't work, then drop the drug -- it's not worth risking your dick over & there's alternate (albiet, scary -- ex: RU & CB) solutions out there right now.
i'd also suggest seeing your physician to get some blood tests done: estrogen, DHT, testosterone, free testosterone, iron, & thyroid.
a lot of times too (i'm not saying this is/isn't the case), you start to worry about it so much, that there's no way you're going to get erect. if you're trying to fap and all you can think is "please work, please work, please work. omg, i'm ****ed from finasteride", then it's simply not going to work. i'd suggest following what i said above & then stay off the forums for a bit -- i've been there & reading horror stories only makes things worse...your brain is a funny thing & can easily psych your physical body out the more you worry about it.
also as a semi-side-note, if you're experiencing "some brain fog", then it sounds like it's not the "brain fog" that some of us actually experience from this. if it's debilitating as to where things feel like you're in a constant half-dreamlike state, and you don't even feel safe driving, then that's "brain fog". if it's a "i don't quite feel right today, and a little sluggish, or aren't as sharp as usual", then that's just you having an off-day (it happens to EVERYONE -- on and off propecia). so if it's the latter, then keep your mind at ease knowing that what you're experiencing isn't from finasteride. if it's the former, then lowering your frequency (as mentioned above) can definitely help -- it surely helped me.