I had/slightly still have the same problem.
Its anthropometrical genetically based problem for me. My sternum and chondral cartilage (ie: breast bone) is more robust on the left. This gives a larger attachment site for my left pec and therefore has allowed it to develop and grow quicker. On the flip side, my right delt was compensating for the right pec.
I tried everything to fix this: extra work on the right, dumbbells, varied grips, dumb-ass elbows flairing out junk, everything. Nothing has ever, nor will ever fix the problem. But, some things that have helped are:
1. Learning to use correct form when pressing: shoulder blades retracted throughout the movement, eblows creating a 45 degree angle with the torso (not flaired out at 90 degrees), back slightly arched, 5 points of contact: head, shoulder blades, hips, right and left foot.
2. Using dumbbells and stopping when form broke on the right (weak) side and my anterior delt started taking over
3. Focusing on movements that DO NOT use the anterior delts (ie: minimal fly work)
4. Strengthening my scapula retractors (rhomboids, middle trapezius, rear delts)
5. Increasing flexibility in my shoulders, especially loosening the pecs with DC shoulder dislocates
Br