I feel your frustration, OP. As someone else who's recently started it (my girlfriend began playing and I decided to follow once it went free since. Turns out I actually enjoy online FPS despite what I've told myself all my life.) here's what I've gleaned:
1: Yeah, you're gonna loose. ALOT. Personally I go through spurts. I'll have a night where the team and me are just ON. and we just steamroll through the opposition. Or we'll still be winning, but it'll be a fight and we'll all be grinning through every second of it. Or some nights...well, some nights I just can't catch a break. Spies and Snipers everywhere. It just seems to happen.
2: the Ctrl button is your friend. Cover is your friend. ESPECIALLY if you play as a Medic. I can't tell you how many times I've been on Nucleus, and me and a Heavy have been trying to capture the control point and the Heavy just rains bullets on people as I'm crouched behind one of the walls, cut off from most people aside from Snipers, and heal him frantically and pray for my Ubercharge meter to pop to full. Curiously, those moments always tend to end up with us capturing unless someone is brave enough to come out and kill us close-range. Or more specifically me. Crouching and cover in general is invaluable. It's all over the place, and you can use it to your advantage.
3: Everyone dies. Alot. When I die, I usually take a moment to hit the change class button. I can see who my side has in terms of classes. I can see if we've got a Medic or not, I can see if we've got Pyros or Spies or Engineers. Then I combine that with what I've been observing on the battlefield. Snipers pegging everyone but we have one Spy? I respawn as Spy and pull out the ol' Ambassador and start handing those Snipers some of their own medicine. No Medics on the team but alot of Heavies? I roll a Medic and promptly find someone big and vaguely russian with a big gun to follow around. I go where I'm needed and do what's needed if I can. Noone has yelled at me yet for it.
4: If you feel like items, or lack thereof, are an issue, the Starter Packs in the Mann Co. Store are GREAT for that sort of thing. Especially if you try out every class and you focus on finding which one clicks with you and then buy their starter pack. (of course depending on your luck and how long it takes you might have advanced items for that class anyway!) The way I figure it: They're a buck, it's not a bad deal, and I like the game. So if me dropping a buck or two for a weapon pack for Medic or Sniper helps keep the servers and updates rolling, I'm fine with that.
5: HAVE FUN. Bottom line. Worst thing you can do for TF2 is go into it with the 'I don't have fun unless I'm winning.' mentality. I've had some pretty epic deaths, and some pretty hilarious losses. There's really no official server rankings or anything that I know of, so it's not like some games where you're under constant pressure to perform better and a loss can actually affect your gaming experience. Losses are just losses. You still get items, you still get to play. If some jackasses want to boot you off their server because they think you suck? Let 'em. Plenty more out there to enjoy. Heck, as I recall the Escapist has a TF2 server.
Hopefully that helps. I know it doesn't seem like much, but I probably don't have more than 20 hours or so in and I've had a blast. Nerfnow is correct: it DOES get better.