On age and fertility: It is a very individual thing. IMO much of the scare-mongering is invalid. Most fertility clinic protocols are based on a very small number of studies done a long time ago and it just isn't true that getting pregnant at 40 is impossible. See
https://www.theatlantic.com/magazine/archive/2013/07/how-long-can-you-wait-to-have-a-baby/309374/?single_page=trueI mean, ask your doctor whether being 40 is birth control and they will look at you like you are nuts and prescribe an IUD. Women over 40 have some of the highest unplanned pregnancy & abortion rates.
My great-great-grandmother had her 7th kid at 44. I had a kid at 35 and 44 -- no treatments, all natural, pregnancy went fine, no problems. (The only thing I did was regular acupuncture for more than a year before #2, in order to lengthen my cycle which had already started to shorten.) I have a friend who had her last kid at 43 -- again, all natural conception.
You can expect more miscarriages the older you get, so don't be surprised if you have 2 or 3 or 5 pregnancies before one takes, but. (Don't forget that around a quarter of all conceptions at any age end in miscarriage. Many too early for people to really notice, but.)
Also, the freakout over genetic abnormalities associated with age are greatly overblown as well. Yes, there is a slightly higher risk but it's like going from .5% risk to 2% or something like that. 98% chance of normal. (And many of those fetuses will miscarry) You're not suddenly 90% likely to have a baby with major issues just because you turned 40.
The dangers of being an older mom to your own health are also overblown, especially if you are very healthy and active. (If you are not doing a lot of exercise now, start.) However, you WILL be at risk of being labeled "high risk" by your doctor and be pushed into things like C-section at 38 weeks for no reason other than your age. (This is one of the reasons the US c-section rate is now around 40%, an insanely high rate. It should be around 10-12%.) For this reason, if you have no other risks besides age I highly recommend getting your prenatal care and doing labor & delivery with a certified nurse-midwife -- there should be some working in hospitals in your area. I had both my kids with midwives in the hospital and even when I was 44 nobody batted an eye or ever even mentioned my age. That would NOT have happened with an OB.
Pro tip: Get your fallopian tubes flushed out before you try to conceive -- a hysterosalpingogram . It's meant to diagnose blockages in your tubes, but something about it makes conception easier even if you have no blockage. Both midwives and an OB told me that anecdotally they see this happen all the time (and it certainly worked for me).
Also: Get a copy of the book Taking Charge of Your Fertility. You should be monitoring your cycles to check if and when ovulation is taking place and if the various phases of your cycle are long enough to support implantation etc.
And since this is MrMoneyMustache I have to add: If you use Taking Charge of Your Fertility and everything is good with your cycle, you should even be able to do the low-tech turkey baster DIY insemination route if you want. You can have sperm delivered to your home via FedEx, I believe. (Seriously.)
There is no need to jump right into uber-medicalized conception if there is nothing wrong with you. A fertility clinic will tell you it's impossible to DIY, of course -- but it isn't. (I know at least one kid conceived that way...) In fact the lower stress involved in DIY might help you conceive faster. Most fertility clinics have very low success rates -- just like when you do it the old-fashioned way.
On the subject of single motherhood of infants: I did it with my first, and not by choice. Put away as much cash as possible and assume you won't sleep more than 4 hours at at a stretch for several years after baby is born. $600/month for daycare for a baby is ludicrously low unless you only need about 6 hours a week. In major urban areas you can assume more like $2000-$3000/month. (and the good places have waitlists that are years long. You may have to sign up as soon as you are pregnant.) A nanny will cost at least $20,000 a year half time. (In Boston, even teenage babysitters charged $20/hour.)
Babysitting co-ops (my fave form of daycare) are fantastic if you can make them work, but if you thought caring for one baby was hard, try doing yours and someone else's all afternoon. It's a lot easier to do them when kids are at least 2 or 3 years old. Getting a live-in au pair or nanny is probably the best if you can manage it. If your mom can live with you for several years and can stay healthy during that time you will be very very lucky.
I also suggest finding a way to work PT for the first 2 years at least -- yes, your income will be greatly reduced but it will save your life, you won't have to spend as much money on child care, and you will actually get to raise your own child (mostly).
Not sure what you do for a living but part-time professional jobs with full benefits do exist. Look at colleges & universities, hospitals. Once you are PT, get an income-based deferment on your loans and try to forget about them for a while. Pay the interest so it doesn't capitalize, but otherwise another 4-5 years won't really matter.
Assume your income will be greatly reduced for a period of years (possibly forever).
If after kid is born you feel you don't want to be away from her, you could also become a nanny yourself. It's harder to find someone who will want a nanny who brings their own kid, but some parents like it because it gives their kid socialization. You probably won't be able to charge as much, but you can then make money and be with your own baby all day. I'd guess a FT nanny in DC could be making around $40K.
Or start a home daycare with your mom -- you could probably gross at least $60-$70K a year doing that. Might be a lot easier once kid is 2 or 3 years old though.
Or move to Canada, where you get a year of paid maternity leave and then high quality child care that costs like $50 a month. (Or almost any other democracy besides the United States, which rates near the bottom of the list for child/mother/family conditions in all categories. It would be vastly easier to be a single mom in say Sweden.)
Finally: Any chance you are friends with a gay male couple who might want to have a kid? You could have a baby with one of them and you could all help raise it. Get a duplex and you live in one side and them in the other, and kid goes back and forth like you're divorced but much friendlier. Again, not actually joking about that. If you don't have family who can give large amounts of help and support you need to create one. Babies and new moms were not meant to be left all alone.