A couple of comments.
1. I have found that communication is the most important part of sex. Talk to your partner. Find out what she thinks you are doing well, and what she would like. Tell her what you would like. If you are worried she could lose interest... talk about that.
2. Condoms make it more difficult to keep an erection. I am going to be irresponsibly honest (people may get upset but this is the truth). You have to weigh the risks with the benefits (and there are real risks), but many people ditch the condoms once they are in a stable monogamous relationship.
3. You don't have to be motionless when you are on the bottom. If this position works for you... experiment with thrusting with you on the bottom.