Nah, you can do it. You were just making some mistakes and need practice at it. Permanently sheeping that late game carry that you were having so much trouble with is incredibly satisfying. Some things I noticed.
1. Practice last hitting in an empty lobby. You don't need a Null Talisman, and at most a Mantle of Intelligence. It is hard, and weird because of the projectile speed. You just have to be anticipatory about it, especially against high damage opponents.
2. Stack ancients. I watched one match where you were on the offlane and weren't getting much farm, and you could have easily ran back on Dire side and stacked the ancients. You can stand on that little spot on the cliff to the left of your T1 tower and use March from there.
4. You have to pick and choose your skillbuild. You can't really do 1-1-1 on Tinker with any effectiveness. Instead either go 4-4-0-0 or 4-0-4-0. The latter is generally better as it allows you a way to catch up (Marching ancients), but the former is better if you are snowballing. I find it better to max laser as it delivers pure damage and the blind effect has it's obvious perks.
5. Get a bottle, even if you aren't rushing it and it's after BoTs. Part of the reason people rush Bottle on Tinker is that you spend less time when in the fountain if you keep spamming it, and you will be there a lot.
6. Dagon rush really isn't that good. A Blink Dagger and Sheepstick are both far and away more important. Dagon is just a lategame "for funzies" item. When you buy Blink, make sure you Blink into the trees and march from there!
7. Always Soul Ring after a Rearm.