Working on a project for 18 months doesn't give you enough insight into it to know what is good or now about it. You need several more year before you can usefully figure out what changes will help get you to make milestones. (other than trivially obvious things, which might be the low handing fruit - but sometimes they are the better way to do things but the real problem makes that stand out instead).

Nothing will get you to hit every milestone. However you can make progress if you have years of experience in that project and the company is willing to invest in the needed time to make things better (they rarely are)