I would really encourage the OP to speak Spanish to your DD if you want to, errors and all. I speak non-native French to my son, I know I make errors but I also know I can communicate, which many people can't, despite spending years learning a language. I started late (he was 2) but he understands native speakers with no problem and can make himself understood in an all French environment. He watches TV in French, has bedtime stories in French, and we sing nursery rhymes in French.
I couldn't really disagree more with people who say that because you won't teach your child something 'properly' you shouldn't do it at all - there are many many examples of adults and children who have been taught languages 'properly' at school but cannot string a single sentence together. My son (aged 3.8) already speaks French better than most GCSE level students I have heard, and whilst he does make errors, he makes them in English too and nobody finds that a problem. I would rather he makes errors in 'le' or 'la' but can communicate than not be able to talk at all.
This thing about speaking Spanish to your child in an English speaking environment is that it is additive, not subtractive, in which case the 'golden rule' of 'speak your own language to your child' does not apply at all. What that means is that your baby will definitely learn English, whether or not you speak English to them, there is enough exposure there already. If you speak Spanish to them, they may or may not learn some Spanish in addition, but one thing is sure, it will not prevent them acquiring perfect English. If you were Spanish but decided to speak English to them because everyone else was, that would be subtractive.
The other fantastic thing about speaking a non-native language to your child is that you have a wonderful opportunity to improve in that language and to make it part of your everyday life. When I started my French was not that great, but the language you speak to children is simple and you can study to keep ahead of them. I have French lessons, listen to French podcasts, watch TV in French and read in French. Learning French together has been one of the most rewarding things I have done with my children. I would really encourage you to go for it - the more you do it, the more natural it feels. It is not a cheap option, you will need to buy DVDS, CDs, books etc in Spanish to keep the exposure up.
Before I stop, one book I would recommend to a non-native parent is
'Play and Learn Spanish' by Ana Lomba - it has lists of useful phrases for various situations so provides a crib sheet. Also, google 'non-native bilingualism' to find lots of blogs of people doing this, and lots of encouragement!