I will bet you have more project management going on than what you are giving yourself credit for. The natural tendency of any group of individuals embarking on a task is to form a team, establish roles including leadership, establish rules, establish processes, and figure out solutions. Since this has grown from one week to six months, it shows that this must have occurred, else your team would have imploded and you would not be asking this question.
That it is undisciplined, informal, and immature from a capability perspective might be true, your project is being managed.
Two alternatives come to mind: 1) Take what you have and begin formalizing. This is the first step in growning your maturity level. Document your processes, establish documented roles and responsibilities, create your charter, formalize your requirements, formalize your project controls. The advantage to this is you are taking advantage of what has naturally started without stopping your project. The disadvantage is, growth is slow moving, some resistance can be expected, and performance may degrade for awhile.
2) Stop everything. Grind everything to a halt. Start over. Re-establish your charter and go from there with the intention of establishing better project requirements and goals, plans, processes, teams, etc. The advantage here is a clean start, automatically removing any existing bad habits from your previous projects. And you are likely to arrive at a higher capability in a shorter time than with the first alternative. The disadvantage is you are stopping everything. This can cost you financially and politically and whatever else.
I have no formal back-up on any of this, just my opinion on two possible alternatives.