I do not think that missing milestones is necessarily a failure. To me, milestones are sort of like soft checkpoints in a project. You should do everything in your power to make sure that the certain set of tasks is completed at that certain planned date for that milestone, but if it is delayed there is still a large potential for the project to still be completed adequately at the original planned end date. You may need to adjust some of the consequential tasks dependent on that milestone, but it may not even be apart of the critical path and so may not even delay any other activities. There are just too many extraneous factors that can and will derail your project, so you should not be devastated if a small milestone is missed. I learned this lesson in my senior Capstone project. There were obstacles popping up which we never would have been able to properly prepare for, but it all depends how you can respond to that adversity and get back on track.