This is not an a typical issue and rather common within the industry. I may be able to help but here are a few things to consider.
1. Changes to scope and scope creep will certainly add time to a project, so hammer down the scope of work moving forward and allow for no further change requests. Ad the on as a phase2 after the first items ade acoplished.
2. An itemized list is good, but need to start with the objective then break it down into stories, or tasks acompanied by time estimates. Each should bd no bigger than 4 hours and if it is break it down further.
3. Figure out dependancies, what needs to happen before the next can start, a before b kinda thing.
4. Then sit down together and really figure out what you can get done on a weekly basis. Some call these iterations sprints but regardless the name jt will help hou figure acurate timelines.
5. Lastly Allow for error! So there are all sorts of numeric formulas to calculate estimating overages, I lkke to do it one of two ways. First if the developers tend to be overly optimistic or have underestimated before, i DOUBLE the time, yes DOUBLE IT, otherwise I go with 30 - 50% increase. So 10 weeks becomes 13 - 15.
Just my thoughts,