If your lips are getting tight, I would encourage you to relax a little bit more.
If you're using the Deep Relaxed Embouchure, as explained in my lessons then your lips should not be getting tired. ONLY the lower lip very slightly contracts to bring the corners of the mouth SLIGHTLY closer together.
It's okay if you're a little sloppy in the beginning while learning single notes. I'd rather you be RELAXED than develop the bad habits that come with tension (and trying too hard with the mind). So please relax and enjoy yourself and you'll get it in time... I PROMISE!