How can I be more comfortable being squishy?

I've been playing DOTA and DOTA 2 for years now, but I've always had a problem: I suck with squishy heroes. Really, as soon as a hero is even a little bit squishy, I get under-leveled quickly and die immediately as soon as a team fight breaks out, no matter how far back I think I am. Whether it is a nuker or a support or whomever, I can't seem to stay alive long enough to contribute to the fights or a win.

Now the obvious answer here is to practice. But there are sort of 2 problems with that:

1. I've been doing this for years, and I have practiced quite a bit, but I still haven't really got better. I'm not sure if I'm just making the same mistakes or what ...
2. While practicing, if I'm with a team, often people will report me as being bad or be very vocal in their "disappointment". Whereas if I'm with bots, that can be kind of helpful I feel, but bots play very differently than most people do, so even when I get OK with a hero, going into a player match still gets me killed.

How can I successfully counter being squishy, especially early game? How can I get some good practice in without handicapping my team while I learn?