I know exactly how you feel, and in hindsight I'd have never taken on my current project, and would instead have bought one that was further along, or even finished already, and modify it to what I like.
Mine isn't a Tri Five currently, but my '39 Chev coupe. Almost everyone who saw it when I bought it just shook their head and said something like, "What were you thinking when you decided to buy this?"
What I was thinking might be close to what you considered when you bought your Tri Five. It was exactly what I'd wanted for many years, and the price was extremely cheap....for good reason!
My particular car was so rotted out that when you closed the doors the whole side of the car moved in and out as it swung freely! And except for a beautiful frame, everything from about 4" down was rotted away! If it was free it likely wouldn't have been taken by a lot of people!
I became quickly aware of how overwhelmed I was with the project, but also figured I paid so little for it that I considered it a learning process, and likely not something I couldn't sell and get my money out of, or part out and be even if I did. But I began selling excess parts, buying repair panels, and removing and replacing bad sheet metal. It was a long slow process, and some days it seemed like I made backwards progress when I installed metal, and then cut it loose to readjust it, and make it right. There was more than a few times I regretted taking on the project, and that wasn't helped by friends sending me emails with links to identical cars for very cheap prices, that were running, painted, and looking very good!
In the end, (which I'm close to, but not done.) I now have a car that's 100% solid, and no rust at all. It's also brand new engine, and upgraded trans and front/rear suspension. And I love where it's at now!
All I can say is don't quit on it! You're overwhelmed by the project, but if you continue on with it you'll reach a point where you look at it and smile knowing you got it to that point. And if you give up, you'll remember that with a bad feeling inside.
I love being at the point I am now, and having my friends who thought I was nuts, now saying how amazed they are to see it where it's at today.
Just one thing to add. Work on it every chance you get, even if it's only for 30 minutes or an hour! If you walk away from it for long, it's easier to stay away, and get discouraged. The more times you spend a little time on it, the more your attitude will stay positive, and the more you'll want to keep going.