First and probably the most important, I understand. I was there.
Second, there are medicines you can take that have shown little to no results in baby's blood. (I'm prescribed Zoloft in the lowest dose for that reason.) Try talking with your doctor about something like that. Honestly, I'm surprised they haven't talked to you about it already? I don't know any dr's that would suggest anything to take baby off the breast to be ok.
Third, it will pass. While you feel very restricted and limited now, this is part of being a mother and some day your child won't need you like that anymore. I felt exactly how you're feeling. My pregnancy was unplanned and while I wanted to be a mother, being one early in age and when I wasn't prepared ended up making me resent being one all together. When my baby arrived I was excited, but as the days and weeks wore on I began to feel stuck. Like you can't live your life, right? But the weeks start flying by, soon. Right now, especially premies, they say to feed every two hours. Speak to your pediatrician about the ideal weight for your baby right now. I know for babies born on time that as soon as they return or pass birth weight, then they can be fed less often. Which gives you more time to do stuff.
Also, I couldn't WAIT for my baby to be able to start bottles (I wanted to breastfeed and bottle feed so I could go out). But she just had to have one the other day because she was ill and I literally started crying. You really don't know how much it means to you until it's not the only option anymore. What I can say, having gone through it myself, is take it all in. Breathe and do what you need to in order to relax. (Have your significant other, friend, family member or whatever take the baby if they can in between feedings and relax.)
Breastfeeding doesn't last forever. (By six months your baby can start solids.) And it's literally the ONLY thing in the world that your baby can do with you and no one else. You'll miss it when it's gone, so cherish it. (As stressful as it can sometimes be.)
But! If it does come down to you starting formula, DO NOT FEEL BAD. Breastmilk is best for baby, BUT, you being your healthiest is what's best long term. Depression and anxiety are not things to toy with and a lot of people can't relate or understand what it feels like. On one hand you love your baby more than anything, but on the other you feel trapped and stuck even though this is what you wanted. It's hard! But it's only been three weeks, don't give up on breastfeeding yet. It DOES get easier, I promise. My baby is two months old now, sleeps through the night and feeds on demand or every three hours during the day. I thought I would be stuck inside forever and now that I can go out on my own and such, I miss the time I had when she was younger.
(Look into government help for formula if you need to. If your state has a WIC (women, infants, children) program then that may be helpful to get formula.)
Breathe. Relax. And keep trying without forcing yourself or feeling guilty when things aren't working like they should. It'll be over soon and it'll never come back. Try to think of all you have to be thankful for and happy about, that always helps.