Certification wound not be immediately necessary if you had more then your brief parenting experience to pull from professionally.
As the other posters said if you had some other depth of experience in the birth community it would be realistic for you to now work as a postpartum doula. Many of us who don't initially get certified and started working as postpartum doulas were la Leche league leaders, childbirth educators, RN or trained Lactation Consultants beforehand.
You don't necessarily need certification to hold you back.
but you need some type of reservoir of knowledge to offer new parents that is evidenced based knowledge about breastfeeding and maternal mood disorders.
Call up a local doula service and ask if they will train you (I don't know if many do that anymore since the DONA certification is less hassle for service owners). I'm a doula service owner and I used to do a comprehensive training for all my doulas, but most go through DONA now.
Being a mother who breastfed is the right start for being a postpartum doula (you don't need special training to be encouraging, thoughtful, kind, and know how to listen and not judge, you also are already experienced enough to teach someone how to diaper, bath or sooth a newborn, cook, food shop and do laundry!).
However that is just one aspect of the work and learning evidenced based breastfeeding management is what makes this work valuable to new families and what they are paying for so you need more training in that area before you go to work.