Adults sometimes doubt their own learning skills too much. While it's true that our brains never function as efficiently as they do when we're children, adults have some advantages over children where learning is concerned. These advantages can help you to learn French as fast as any child, provided you're using the right materials. For adults, however, it's important to remember what advantages you do have.
First and foremost, adults have patience. Even though you want to learn French fast, you're going to have to be able to make yourself sit down and do the work involved. Adults have a huge advantage in this regard compared to children.
Adults also have a much more comprehensive grasp of their own language. While French and English are closely related, the syntax in French is oftentimes quite different, particularly where describing the properties of an object—that is to say, the use of adjectives—is concerned. Adults, however, have usually heard enough sophisticated syntax in English that they can grasp the idea of having words shifted around without altering the meaning of what they're saying. Children oftentimes struggle with this.
An adult can also learn French fast by tapping into their fun side, something that children usually do a bit better than adults. This can be done by incorporating materials into your lessons that are fun, but also educational. As an example, you can take materials that are reliant on song and story and get rid of some of your phrasebook courses. Singing is a great way to relearn how to use your mouth to form unfamiliar words. You may well have had no need to do this since you were a child yourself, and it helps to use activities that make the effort easier. It's much easier to sing a new word than it is to pronounce it, in some cases.
Adults can, indeed, learn French fast. To do it, however, you may have to give yourself a bit of leeway and allow yourself to have some fun at the activity. While there is much to be said for the advantages of discipline, focus and purpose and adult has over a child, there is also a lot to be said for making studying fun. By combining a bit of both into your plans, you'll likely find that you learn much faster and that you retain what you learn better.