Jessica, it has been said, and i will repeat just because I can. You are 21, and cross dressing for you is mos likely sexual, and you experience the low after sexual release, just like a typical male. It is normal hormone activity, and the low feeds regret only because there is that societal factoid in your mind that this really isn't something a man does. The sooner you really and truly begin to eliminate that factoid, and accept yourself and this activity, the sooner this post release feeling will progress away from regret to acceptance and relaxation. When you arrive at this state, dressing begins to move away from the sexual if you are meant to. If not, it will remain sexual and you will be happy.
You are young and new, and learning about yourself. Your activities are acceptable and not to be judged negatively. Stop and take a breath.
Barbara