Regenerating infestation regenerate to quickly. 10 turns is way to little. Would be better of it was 20 turns or if it was delayed and nothing began regenerating untill like turn 40.
Right now, by the time you have an army for the stronger infestations there are like 5-6 infestations sending raids on you from different places. There is no time to clear them and let your troops heal up again before next wave.