Once you have a plan with detail and measures you should regularly assess where thing are at. The detail should be able to show the cost blowouts, the savings, the time delays.
So once you know where things are at you do not meed to despair. Start with the changes to bring it back into alignment, prune out what is reasonable but do not give away your standards and goals. Reset the plan so you can measure how the changes are going.
Sit back now, reflect on the variances and your revised actions. This reflection may assist in repeating the mistakes and you may find great ideas to improve in the future.
As you may have guessed planning, variances, adjustments and improvements are a continuous cycle for managing any organisation large and small. Managing is not a set and forget function, you need to be super vigilant consistently.
Put your regular assessments and improvement evaluations in place today!
If unsure on this certainly seek assistance from professionals who can help. Do not try to skimp and leave gaps in the management activity.