Hi J.,
The time after a baby is born can be stressful for even the best of relationships. It sounds to me like there are many things on your plate, causing you anxiety and yes, overwhelmed is very reasonable.
I don't know your past, or the past of your fiance, however, here are some ideas:
Join a New Mom's group. Chances are that your birthing hospital, or the local one, runs a program for new mothers that meets regularly. These groups can be vital support for women experiencing the very unique challenges that new mothers face in their relationships with both their babies and spouses/family.
Find a good counselor. New relationships do benefit from counseling, and this is less expensive than maintaining two separate households, if you need an argument to present to your fiance. Relationship counseling is not about changing the other person to fit one's needs, it's about finding common ground in the relationship and having an impartial third party validate both person's feelings while giving tools for open, considerate communication.
I suggest counseling because, if your fiance is already making financial commitments without your agreement, and then expecting you to foot half the bill, there's an imbalance. I think he needs help seeing things from your perspective. Perhaps he's really wanted a family for some time, and is forcing things to work out of fear of losing you or disappointment. The most confident of men would be shaken by the idea of losing their child and fiance, and perhaps he too isn't confident in some aspects of the relationship but feels he must just forge ahead anyway. It seems like there's a 'getting to know you' window that happens in a relationship that you both missed out on. Counseling can help you determine if that bond is there, is capable of being cultivated, or if you want to work toward amicably coparenting. I know it's a lot to throw out on the table, but please consider it.
AND, if he doesn't want to go to counseling, go on your own. If you are living on his paycheck, see if you can dip into your savings. This is probably the most important investment you can make right now; if he's completely opposed, then again--it might be good to find out why.
I wish you and your child the best of lives. Only you can know what will be right for you over the long term. My family was a bit chagrined that my husband and I were together for 8 years, and had a son who was nearly 2 before we got married. We had to decide what was right for ourselves and sort of let everyone else just flap in the wind, as it were. We did go to counseling about two and a half years into our relationship and went for quite a while. It was hard, but has strengthened our bond and our love so much. Self-discovery is hard work, but it's worth the effort.