Intervals are good, but you say your endurance also sucks. So, you need to keep up the saddle time in addition to doing shorter interval workouts. There are also 2 kinds of intervals. Short intervals working on top speed and cardiovascular recovery time and then there are intervals that push your LT such as a 2x20, 1x30, etc.

If you have a hard time really finding your zone for those longer intervals, then keep going to those hard group rides and keep trying to hold on. Sometimes, that type of environment gives you the motivation to push yourself past the limit where, if you were solo, you'd give up. When you can sit in the draft, milk that recovery for all it's worth.