converting from a solid opening to a strong middle game

Anaylse your games and other games. Just watch a few games from someone your level (or a little higher) on chess.com and watch what they do and try and figure out if it was better than what you would have done.

 Computers are very good for certain things. You can takeback, split games to see how positions work and you can get advice on best moves. Just as importantly they can calculate who's ahead at a certain position. You can use that to analyse how far ahead you were and when your score dropped/increased with good/bad moves. Do this especially with games where you thought you were ahead. But don't get the computer to make your moves in existing games. 

 Playing against them is better when you lower their rating because otherwise they're thinking much further ahead than you're capable of following and hence, it's much less likely that you'll learn as much.

 In general, if you're ahead on material, swaping favours you. If you have inititive put your opponent on the back foot. If they have weak squares attack them (not necessariy straight away, you can lure defenders away). If they're in a closed position with bad development, shut them down. Stop them breaking out so they end up spending 5 moves to get a knight somewhere half decent while you've been mounting up. Then when the time is right start your attack. Setting up forks/pins as part of an exchange sequence is a really good way to gain an advantage early in an opening. To do this you should be looking at studying important combinations, forks and pins. Learn them, use them.