Yeah does sound pretty extreme, especially at 25. For some reason I thought you'd be much older to experience this sort of thing, though I guess age has nothing to do with it. I'm almost 45 and been on the finasteride for 1.5 months and the first three weeks the erections were weak AF but are getting better and holding for longer each day.
I'd suggest working on the psychology, so the mental stimulation. I went through a bad stage in my 20's where I had terrible IBS and it was a real boner killer. It started with a really bad night with a girl I just met and gone to her place where her toilet was next to her bed and it had no door! My IBS was bad and the stress of not using her toilet resulted in terrible sex. From then on my mind was convinced the same thing would happen every time I went back to a different chicks place and I really struggled to maintain a decent erection. I think my mind was constantly thinking I wasn't going to get it up and the signals were telling the blood not to go down there. It was only when the IBS went away (or I didn't care as much) things down below went back to normal, in fact better than normal, once I got out of that mindset.
Even now watching prn on its own doesn't do much, though if my imagination is firing off and I'm expecting the boner to happen it happens and lately it's been stronger than ever. For a while on finasteride they were really weak and I was starting to panic, though a brief fantasy ran through my head one night and downstairs went turbo and held for ages, then after that my mind was convinced it will happen again and it did. Anyway, not saying this is the cure but something that worked for me.. so far.