I think the OP might simply be terribly lonely. This "relationship" is the only thing masking that and making it bearable, which is why OP fell into sadness/depression when they were apart.
In that context, I get the idea that bringing a baby into the situation might help with the loneliness; however, as others have said, it's unlikely to work out in the long term. Either the baby will disrupt the current comfortable situation and drive the two adults apart, and/or OP will indeed get attached to the child, only to be devastated later when the mother decides to take a different path in life (even just kicking out her family and moving back home).
OP, my advice to you would be to start now making an effort to make new friends/acquaintances, and also perhaps consider getting a pet.
On the former point, there are outlets such as Meetup.com that allow you to sign up for social events without already having to know people, or commit to a long term club membership, etc. I know it can be painful for a strong introvert, but some degree of social interaction is necessary for all of us, and can lead to unexpected benefits. Look for events and groups that involve your interests; it makes conversation easier, and you're more likely to find people who are fellow introverts and understand where you're coming from there.
In any case, having other social outlets and connections can make it easier to separate your feelings for one particular person from your general need for *someone* in your life. I think you need this extra perspective in your life right now.
On the latter point, owning a pet is a much less costly commitment in terms of lifestyle compared to a baby, and in this situation also carries less potential for heartbreak1, since the pet would be yours. Obviously you would want to research and decide which type of pet is most likely to work for you. One particular benefit is that pets offer companionship without the need to talk and be overtly social, so they shouldn't drain your energy the way people (even little ones) might.
1 except of course, when the pet eventually passes on, but for most companion animals that's a distant prospect, and the intervening years of joy and companionship are well worth it.