We had the SAME problem in early November! Ironically, pre-baby, I was afraid I could never love my 2nd as much as my first....but once that 2nd was born, I started to wonder if I'd ever love the first the same way again since her complete personality change made her so miserable to be around! With all the raging hormones, lack of sleep, and lack of help (my husband works 9-7), I was at my wit's end. Examples of 2.5 yr. old's behavior were (yes, PAST tense!) yelling, tantrums, hitting me, biting me, kicking me, etc. The worst part of this is that prior to the baby's arrival, she'd NEVER shown any of this behavior and was a model child.
Long story short, what saved us were good, old-fashioned time outs ~ with a twist. Because I was short on man-power, patience, and alertness, I turned her high chair into her time out seat. This meant that she'd be strapped in and I wouldn't have to repeatedly chase her to put her back. I mean really, I was already having power struggles, did I need to add another where she'd revel in the attention, albeit negaive? Yes, strapping your child in a seat sounds a bit cruel, but I couldn't continue hating my life and crying every ten minutes because she'd really hurt me physically and emotionally. When I'd strap her in, she'd resist by trying to hit/bite me, which I'd ignore and wouldn't get upset over (eventually she saw it made no reaction and gave it up, but every now and then she tries it again). Then I'd set the microwave timer for 2.5 minutes, turn the chair around so that she wouldn't have an audience (but place it far enough away from things that she couldn't reach anything), and walk away. When the timer went off, I'd return, briefly tell her "We don't hit, that hurts Mommy," ask for an apology and a hug, and let her out of the chair. We'd start from scratch then, as if it hadn't happened.
Believe me, there was a day when she was in that chair 15 times or so, but that was IT. Now, when I see her temper boiling and recognize that she is going to blow, I remind her "If we hityell/bite/etc we go in time out." She's literally had her hand raised in the air, ready to strike, and with that warning has dropped the hand, turned around, and started playing with something else!! I finally feel like I have gained back control and am not at my daughter's mercy.
Also, I was originally afraid that making her high chair her T.O. seat might make her hate it, but she is able to tell the difference and actually loves using the seat to eat. I never used the tray when she was in T.O., so that may have helped. She also has a booster seat at the table, and likes picking which one to use for meals.
I wish you luck and hope that someone's advice works for you. I can completely sympathize as I was in your shoes not that long ago and was to the point where I just wanted to get in my car and drive away ~ or be admitted to a hospital mental ward where I'd at least get some sleep, care, and attention, and not be feeling like my *toddler* was running the show!!! Believe me, I suggested this to my husband after very long days (and he was to the point where coming home was hard, as he'd come home to chaos and crying).
Truly and honestly, my little girl is back to her old self and the storm has passed. It took about 2-3 days for her to realize that I was CONSISTENT with the time outs, and that dreamatically reduced her outbursts. After a week, I had my great kid back!
Good luck :)