Advice with feelings of list

I'd like to start to this out by saying that I CANNOT break up with this girl. I understand that the best option if we were only a couple would be to leave her to avoid her pain, however we were irresponsible and now she is pregnant. I have to do everything I can to make this work and give her the best life I can.

So I (M24) have been dating this girl (F23) since january now and for some time now I get really depressed everytime I see a pretty girl. I just can't stop myself from admiring other women and thinking how beautiful they are. I don't even have to stare at them. A single second glance from a passing car is enough to ruin my mood. I love my girlfriend and she is attractive, but she is overweight as of now. I like to believe that I was honest when I told her I don't mind her being a little chubby, but things have changed and now I feel this overwhelming sense of dread everytime I see a good looking woman pass by because I kmow that that's not what should be going through my head all day... I know my gf is beautiful because I saw her only 2 years ago and she was a star. How can I stop these lustfull feelings from ruining the relationship? Are they normal?