Hi. I do agree with Suzie_Homemaker 100%, but also let's donot forget that children at that age are more sensorial, look in you environment is it overwelming? (Too many colours, high noise ( even music can be disturbing for that age) or too busy) you can try creating a quiet clear spot for the baby.

Children at that age are mostly guided by their senses (reasoning skills still developing) and due to my experience in this field I always recommend first time parents to socialize their babies, sharing them with family members or friends or visiting playgroups and parks (supporting a trust relationship between babies and adults). Also, if the child has stuffed animals or a preferred blanket it definitely helps (tactIle input) and as they are also guided by smells ask mom to leave a scarf or sweater that she wears, it is amazing to see them come down once they get moms smell. Another think is the mom still breast feeding? (Then you need her milk handy to come her down).

Psichological aspects tell us that, it is really important to bond the very first days. Although you are ready to terminate I would personally recommend to try to cover all the basics first and then decide. Get your stuff ready to go, earlier. Hold her as much as she needs too. (To be honest I haven't heard if a child that needed to be held for many years).

Once that baby has developed that bonding and trust then she will be ready to explore on her own, knowing that she is secure.

So far what I determine through reading your post is that the baby has developed already trust on you (that's why she cries when you disappear behind a door), there is a term in child development called "object permanent" babies and toddlers
Brains still developing the skill to figure it out that people or objects are behind a door. (They get scared they trusted and suddenly it is gonne). I hope you give a try. Once that baby bonds to you that baby will grow and trust on you completely you won't even remember this episode.

If you have playgroups visit those so your group have a break and you can focus on the baby. Or just simply take them for a walk.