Assuming You Know What Your Partner Wants
We often assume that what feels good for us will feel good for our partner. But that is not always the case. Make sure to ask what they like and check in with them. Pay attention to the cues they give you and be aware of the fact that they may not enjoy the same kind of touch and stimulation that you do.
Criticizing Your Partner
Intimacy involves vulnerability, so to criticize your partner about intimacy issues (especially while you are being intimate) is likely to string pretty badly. Make sure to discuss concerns and desires in a non-judgmental and non-critical way.
Focusing on Your Body Insecurities
It is normal to have some body insecurity. And it is wonderful if you don't have any at all. But if you do feel insecure, focus on it during intimacy can kill the moment. Focus on your body sensations instead of how it looks. The best way to enjoy intimacy is to be in the moment and let go of any shame of fears you have around your body image.
Mentioning What A Previous Lover Did
Mentioning what a previous lover has or hasn't done in the bedroom is only going to lead to hurt feelings. It is important to communicate what you want, but don't bring past relationships into the discussion.
Expecting Your Partner to Read Your Mind
You may think it is obvious to your partner what you want, but don't make that assumption. Even if you have requested something from your partner in the past, don't assume that they remember. Make sure to clearly communicate what you like and don't like and don't expect your partner to read your mind.