Every time I have sex with some one I cannot get on top... And when I have tried I freeze up and can't move...
What's wrong with me??

Purely a mental thing. Are you comfortable enough with yourself to let yourself go with the moment?

What will u be thinking when this happens?

There is NOTHING wrong with you. Try having your man lay on the couch slightly elevated, then you climb on top with one foot on the ground and the other one on his other side. This will give you great leverage plus it brings him closer to you so you can kiss or he's right at eye level with your tatas. Then just ride away. Men LOVE getiing to relax a little and watching you get off this way. Confidence is key here. Live in the moment. If he didn't think you were sexy he wouldn't be having sex with you;)

There's nothng wrong with you. Ive had that same issue its like you feel embarrassed or weird about it. I think it depends on the type of feelings you have for your partner and how comfortable you are with him because I have a fiancé now and I have no problem getting mines on top.

The "with someone" not my partner, not my boyfriend, if you are having a lot of partners for only sex, it will may be a issue of the relationship itself.

But it could even go back to something you were told as a child about sex positions.

Do you mean your knees lock by any chance?