This may be due to stuttering, where focusing on just "saying the words well" causes you to neglect articulation. Not articulating can also be a way to avoid stuttering (you'll have less trouble saying a sound if you say it half-heartedly) I don't really have any advice to give to help you with your articulation, other than seeing a speech therapist, maybe practicing reading by recording yourself can help you work on your articulation, but I've never tried for myself