Once you've decided on which part to work on, crank up your amp to get the same level as your rythmn part. Practice a little that way. Then gradually turn down the volume of your amp (lead guitar). Try to compensate the lack of volume by adjusting your picking attack. Gradualy turn the volume of your lead down until you get both guitar at the same level and that your lead sounds right.
Here's an exemple :
Start at:
Rythmn at 6
Lead at 10
After go to :
Rythmn at 6
Lead at 9
Then :
Rythmn at 6
Lead at 8
And so on...
It's just an idea, I never had that problem but this is how I would approach the problem.
Remember to concentrate on the weakest part of your solo, otherwise you'll get discourage. You'll improve faster with smaller parts. And always remember you want to improve your technique and not your solo, try other parts of other solos too..
Tell me if it helps. There's a lot of teachers in here, some may give you good pointers too.
