You've gotten a lot of good advice already. I have just a few thoughts to add.
Your health--I'm not familiar with HELLP syndrome but I know my sister-in-law had pre-eclampsia with their first son. She was in a coma for a day or two but came out of it fine with no lasting effects. Their second pregnancy, which would have scared me to death, came off without a hitch. So, as many have said, each pregnancy is unique. Check with your OB to see what s/he has to say about the health risks and chances for a second pregnancy.
Two children close together--What is ideal? Who knows really? My parents had 5 kids in 6 years back in the 50s and me...five years later. There are pros and cons to ANY kind of spacing. The close ones provide ready friendship; the further ones provide an opportunity to be more individual. Fortunately, you sound like you're young so you will have reserves us older moms have long lost!
Not being married and pregnant--only you and your BF can decide what works best for you. If you want to be married, that's something you can try to spell out to your BF as one woman mentioned, depending on whether it's your priority right now or not. If one or both of you don't want to get pregnant again, that's something to solve together after you've talked about this pregnancy. But make sure you talk about it before this baby is born so you're prepared the next time around. There are lots of choices out there--for one, the other, or both of you!
As an aside, before you think about marriage, is he a loving and supportive partner? Is he a loving and supportive father? Can you envision your life with him 5-10-15-... years down the road? Can he envision the same with you? Are you ready to be a parent of two whether or not he is part of the picture? These are questions that might be helpful to ask yourself.
I have a niece who got pregnant at 17 and married the guy even though he was getting someone else pregnant at the same time. By the time their second baby came along, roughly 15 months later, he and his family were talking her into giving the baby up for adoption. She didn't want to consider abortion and things were already rocky with the relationship.
To make a long story short, her parents adopted the 2nd baby, she eventually divorced the father, and she is moving on in life at the age of 26. The two babies are now 6 and almost 8 and live nearby. The girls love each other and know they're sisters, even if they also know they have different moms because of the adoption.
I bring up this example to say marriage isn't everything. It's the right person that counts...call me a romantic, but I really believe it. I've known some couples who have had a 1-3 kids first and later had the wedding. They were the perfect match for each other and the children added a delightful addition to the wedding. Others who got married first, sometimes ended up in nasty divorces or were lucky enough to be the right match. A little soul-searching goes a long way when it comes to deciding whether or not to marry, let alone have kids with this person. Meanwhile, some delightful young children are brought into this world, regardless of whether or not the parents stay together.
Religious family--I grew up as a preacher's kid, where the church was a big part of my family's life, and yet I also know things sometimes come out of order these days. In my book, "family" loves and supports each other even if they disagree with the methods, or order, sometimes. (They can also piss you off and disappoint you but hopefully they still stay connected.) Marriage, children, family, religion--It comes down to more of how you feel and what it means to you and your relationship with God. God is much more forgiving than us mere mortals!
Talking to your BF--notice I have this last? He is definitely part of the picture and I hope part of your support during this time. You both created this baby together and may need to work more together than you ever have in the past with two young ones and a tight budget. But if you have taken the time to sit back, maybe light a candle, and think about the joys and concerns about this situation, you'll be better prepared to talk with him.
I hope this pregnancy becomes more of a joy than a concern in the months to come. Best of luck with making this reality a reality to everyone else! We're always here if you need more support. Meanwhile, I like what one person said. Make sure you have someone to hug in person during this time as well! Good luck!