HELP! I spoiled my baby

I'm a first time mom, my baby girl is three months old. I feel like I have never put her down. I just couldn't help myself!! But I've realized that she's beginning to manipulate me. SCREAMING as hard as she can if I put her down or in her car seat. She'll be happy to play on her play mat or on her tummy, but once she's done, she's DONE. I can't stand to listen to her scream like that, it kills me inside! I have an overnight trip in three weeks, then a wedding to attend the following weekend, I need to break this habit of hers so she doesn't stress her grandmas out! Help me, please!!