Condoms causing problems to my erection and new partner

I recently have become intimate with my third partner so I have had sex with two other people before this and to properly understand this I guess Ill run over what that was like. My previous two girlfriends were on birth control so I did not have to wear a condom. I would perform very well and they were always very pleased with sex. I am not trying to have a big head but they always loved it and I felt as if I really did a good job. But with my newer girlfriend who has only had sex a few times, I have to use a condom. I have noticed sex doesn't feel nearly as good and my erections kind of take a hit because of it. I dont stay hard very long. Im not sure if its the process of stopping and having to put it on but regardless even after I put it in its hard to keep. Im not sure if this second problem is related to this but it doesnt feel as good for her, infact she said she barely felt anything. She doesn't seem to like it fast and hard like my previous partners from what I can tell but im not sure. Doggy she said hurts her or gives her this "uncomfortable feeling". Im not sure if I am doing anything wrong or what I should do about it. I know its early and I probably need to learn more about her but some advice would be a huge help because it is actually very frustrating that I cant solve the problem