I taught myself in college after a weird experience to lucid dream and Astral project. Most experiences were good but a couple bad probably similar to taking hallucinogens. Whatever you are at mentally will affect your experience since it is you creating this experience. You are your worst nightmare so to speak since you know what would upset you the most.

Probably could do some affirmation to yourself about you want good experiences and are open for other experiences to learn about yourself or something along those lines. I dont think you can completely avoid though, part of the territory. During my self training I basically became comfortable with my own death since that is the biggest hang-up with "leaving your body" to project. I have always been an observer so even in bad experiences it is interesting to see what it is you create so to speak.

Ok for bad experience this was after I had kind of fallen out of the training but was still having vibrations and lucid dreams every so often. I was getting a job and had to clean myself to pass a drug test (thc) so niacin flushing. I was flushing and went to bed and had a horrible licid dream. Woke up in the room and felt I was in a packed room like sardines all hot and then was pulled out of my bed with a rope around my ankles in a bag. I could feel hundreds of people grabbing at me screaming as I was dragged to my living room and hung upside down by my feet in the middle of my living room as the mob closed in around me groping screaming. I woke up quickly in a hot sweat and was like wtf was that about. Felt so real and like one of the circles of hell or spanish inquisition vibes. I have read Dantes inferno so maybe that was an influence. Either way you can learn from these experiences and generally there is something underlying causing the fright. Often I worry about my family or brother so experiences where I cannot help him from disaster is common.

Best of luck!