Great thread.
I agree with the concept of focus in your development. I've been playing about 4.5 years since I started back. I quit for 15 years before that. Don't do it. I really wish I would have just kept a bass around and picked it up occasionally.
The key to improvement is to use your mind. Think about the weak points in your playing and RUTHLESSLY attack your weaknesses. Don't waste precious time playing in your comfort zone. Try to break out a bit every day.
I also have a rule that no matter what happens during a day, I WILL touch the bass and work on something every day no matter what.
Do you watch TV? If so, do you have a bass in your hands while doing it? You can work on all kinds of repetetive excercises while watching TV.
Granted, it's not as good as just focused practice, but it is time with a bass in your hands. I got this from Brian Bromberg. He was a bit sheepish when he admitted that he spent tons of time with a bass in his hands watching TV.
Maybe you just need to change your focus a bit?
Focus on small goals. E.g., instead of saying "I'm not successful as a player until I can play like Flea", why not say something a bit more empowering to yourself like "I will practice slap excercises for 30 focused minutes everyday".
This is clearly a goal that you can reach without killing yourself. Remember that the accumulation of small achievments adds up to MASSIVE results when practiced consistently over time.
My other advice is to work on your groove with the metronome. You can get by without massive chops if you can groove. This will give you time to develop chops and still find some people to play with which will improve your playing as well. Remember too that no matter how well you can groove, you can always groove better.
Finally, I would offer this. Just recognize that there is no substitute for TIME. It took me forever to realize this.
Oh yeah, one more thing. I felt very close the the way you did when I hit the 3 year mark. I almost took up the drums again. You should power through it and remember that it is not a linear curve. I made huge progress from year 3 to year 4. I'm in a band and getting ready to gig soon.
No matter what, don't give up. Usually the prize goes to the last one standing. Persistence pays!
Good luck and stick with it.
GS.