Have any demos? It would help if you went through them after a match to see what you're doing wrong or have someone review them.

A lot of inexperienced demos end up trapping way too much, which wastes ammo and forces you to hit hard to hit pipes.

People can feel free to correct me here but since the Demoman's damage nerf not to long ago it's put extra emphasis on your longevity to be able to make an impact. Obviously before the nerf it helped to stay alive but you can generally find an improvement in your stats by trying to practice conservative play if you find yourself overextending and being overtly aggressive to make damage.

But this is just general advice on aspects of Demoman you may or may not be having trouble with, I very much recommend posting a demo so people here can constructively critique you.

try getting to mids early and hitting those first few stickies on the first people to arrive. generally youll be throwing a sticky at the other demoman or a spot that scouts often go to. those first few stickies can be crucial and possibly pick people off super early in the mid fight.

dont play demo its a dumb class.

its not dumb its just harder than most classes.

if you just dont do stuff just because it is slightly harder than the other options you probably aren't going to get very far in anything

Charge your stickies, especially on mids. You spend as much time charging one sticky that you hit instead of two uncharged ones that you miss.

defensive stickies when people try to get aggressive on you
recognizing when no ones paying attention to you so you can get aggressive and dump on people
locking the other team out/spamming the shit out of them when they try to follow up on aggression
not fucking up your teams ubers by being in the worst position possible

ive never seen you play but id say those are some of the most common things that demos can fix/do right and see a spike in effectiveness

step 1: stop calling yourself shit

if you can find a mentor or someone of a skill level that you can trust to look over demos with you or watch your scrims they can probably give you advice much better than people can give you based on a self-evaluation through text so i would recommend trying to find someone who can help you, try on the ugc forums maybe?

use le epic boots of invincibility

you probably shouldn't take anything away from your stats in lobbies
they tend to be very disorganized and people will bait you/let scouts hold w and run at you all game
to elaborate on what hellbent said you're never going to improve if you keep putting yourself down and thinking that you're bad
once you are able to develop the right mindset and are able to play the game without being outcome dependent and calling yourself shit you will see yourself improve drastically
playing demo especially at a low level it can be super hard because you need to rely so heavily on your teammates to follow up/support you
obligatory ddk champion's mindset post https://www.youtube.com/watch?v=ECcokwYv9vY
this thread has some useful information on the topic
http://www.teamfortress.tv/12754/i-get-too-angry-depressed-when-we-get-rolled

I actually enjoy playing demo at a comp level. I only put that because i knew demo was harder than most classes...

This is your problem, 90% of demoman dm is about stickies, 9% is about hitting a pipe after you've used stickies to herd them into a predictable movement pattern, and 1% is hitting god pipes with no stickies loaded or whatever. Positioning is important too, but whatever.

Use stickies. Practice stickies in mge. Practice using stickies to make your enemies move predictably so that your pipes actually have a chance to hit. If someone calls you out for trying to improve your demo dm, they're likely trash at the game. Just find another opponent or arena and keep trying to practice. Stickies are your primary weapon, use them as such.

One thing that I found that helped me was the mentality of all of it. This may be just for me but I've noticed that when I'm tense and in this mental mode that I can't really describe (you know you're there when you're there) but make sure you're relaxed in things like fights and such. It's an intense game(mode), but that doesn't mean you need to be tense as well. It took me a long time to overcome that mentality but when you're do, things start to get better. Again, this is my personal experience and you might have something different, but that's what I've learned in my experience

1, when you die you are just sending your team to trash can. never die.
2, playing passive all the time is better than playing agressive all the time.
3, HOLD MOUSE BUTTON WHEN LAUNCH BOMBS. ammo managment is a big thing for demo, so when you shoot better make it worth it.
4, stickytraps. everywhere.
5, dont take solo ubers and try avoiding getting ubered at all, you are the only class that can do major damage without putting yourself in the front except sniper
6, try watching pros play and instead of thinking about dank memes while you do it, try and ask yourself questions like "why is he there and what is he achieving with his play"

Dont post its a dumb thing to do

Well it's not dumb your just shit at it.

WARHURYEAH has a great website that I used when I first started playing back in iron to learn demo, It is a wonderful resource and is definitely worth reading through.

https://warhuryeah.wordpress.com/