Having a hard time with toddlerhood. (Vent)

I am not coping well with toddlerhood. I miss being a stay-at-home mom with a baby. I work full-time now and my son is in daycare. He is 2 years old (stopping counting months cause that's too much for me). He is 2 and I do NOT do well with his tantrums.

Ever since he was born I made a promise to myself to be the best mom for him. No hitting, no yelling, no punishment as discipline, and no shaming. I grew up in a domestic violent home with constant hitting and shaming during tantrums and emotional outbursts. So this promise has been very important for me. I read about gentle parenting, attachment parenting and responsive parenting as I'm becoming a mom I never had.

Whenever my son has a tantrum, I sometimes do really well!!!! Very patient, loving, understanding, getting to his level and helping him regulate. But other times when he spends the whole day pushing my buttons or I'm depressed/anxious from external circumstances, I do NOT respond well at all. I just leave. I leave him crying/throwing his tantrum. I leave cause I'm fighting the urge to scream. I leave cause I'm fighting the urge to say something hurtful. I leave to regulate myself. But sometimes it isn't enough. I come back and I'm still on high alert. I'm still overwhelmed and overstimulated. I don't know how my partner does it. He's so serene, calm, and understanding to both of us. He keeps us balanced cause of our strong emotions and reactions. I feel like a miserable parent and I probably shouldn't have become a mom in the first place.

So so so many times I have whispered to myself "shut the fuck up" whenever my son has a tantrum. So many times I have thought of just yelling back "you're too much for me handle! Just shut up". I fight this every single day. I have never yelled this or said this to him. I can't help but feel terrible for even thinking it!!! I wish I was my partner. He's the perfect parent. I'm the fucking mess.