Greetings, welcome to the Forums.
A photo or two would be of great help here. Anyway, first several questions: Is this Crepe Myrtle (Lagerstroemia indica) a shrub or a standard 'tree'? Is it a grafted cultivar? If so, do you know the name of the cultivar?
If your Crepe Myrtle is a standard, or if it is a grafted cultivar of any shape, you must continue to regularly remove all suckers, so that the plant's growth is not focussed in theri direction. If your plant is a shrub on its own roots, you could consider rejuvenating the shrub my removing the older growth and allowing the new branches (not truly suckers) to take over. In any case, mulch your Crepe Myrtle with a nutritive compost, as well as wood chips or bark. Also give it a further application of a complete fertilizer in Spring. You need to both encourage vigorous growth and make certain that such growth is directed to right part of the plant.