Some emotions are more difficult to deal with than others. It's natural to feel sad and upset when leaving your child and returning to work. The question of whether to stay home or work is an important one, and can only be answered by the parent — every family's situation is different, and there's never a right or wrong.
If you're constantly doubting yourself and it is negatively affecting your home and professional life, it makes sense to reevaluate your situation. Think about why you're returning to work, and consider whether there are any options that could make the current situation more bearable. Also, try putting the time and energy you spend feeling guilty towards thinking of creative ways to spend time with baby. Try creating a schedule to ensure that you and baby get in some amount of quality time every day, even if it's a only a few minutes reading a book together. Remember, parenting is about the quality of time you spend with your children, not the quantity.
It's also important to remember that working and being a mother means you basically have two full-time jobs. It's equally important to schedule time for self nurture. Since you've only been back at work for a week, you're still in a time of transition. It's important to allow yourself to express your feeling. As you settle into your routine, the guilt and sadness around returning to work should decrease.