That happens to me all the time, literally every painting session, and I’ve been painting for 30 years! I still learn lots at workshops, here on this site (ArtBytes), sites like ArtistsNetworkTV, in classes and from other artists. Don’t give up. Just enjoy the process. It’s a challenge! Consider your supplies as well - buy the best you can afford: artist grade paint, good substrates (try Ampersand Gessobord) and the right brushes for the job. Buy a copy of Plein Air Magazine both for the articles, the how-to section, and great lists of workshops. Many people at the workshops are beginners. I met quite a few folk at the Plein Air Convention in Santa Fe last spring who were rank beginners, having less than 10 paintings under their belts. Best of luck to you and happy painting!