I know a bit about this process, having been a disabilities consultant for 30 years, before retiring.
It has become increasingly difficult to get SSDI. There are two major reasons, neither of which apply to you!: 1) they've gotten decreases in funding over the years, and so are sorely understaffed; 2) many people apply who are not eligible to get funds, and it takes time to weed them all out.
You say you're downloading the forms online. **Make sure you fill in every blank, even if it's with something like "don't know", or "doesn't apply". They hate to get forms back with blanks; I know people who have been denied solely because they've left a blank or two.
Make sure you make copies of everything you submit, and everything your doctor(s) submit on your behalf. They have a bad habit of "losing" stuff, and if you have copies, you can move the process forward more quickly by simply making and submitting a copy-of-a-copy.
Before you submit anything, call the local office and inquire about whether indeed you are even eligible. They'll ask for your social security number and be able to check that immediately, and let you know. It's likely you are...but calling them will cause them to open a local file on you -- this is a good thing. Very often the local folks are more open to your application than some bureaucrat "somewhere in cyberspace". They talk with you, lay eyes on you (if you can go in, that's even better), they're more sympathetic.
Make sure your doctor(s) knows you're applying. He'll be asked for his "opinion", and your records. If you have more than one doctor, let all of them know. Be prepared to dig around for all the medical records you can get to, going back as far as you can.
As you fill out the form, don't lie...but don't gloss over stuff, either! I tell people, "think of your worst day(s), and fill out the form, reflecting that." We all-too-often tend to say, "Oh, it's not so bad!!!" when we're barely able to move! Don't do that!
If you should get denied, don't despair! Then would be the time to hire an attorney to work with you through the appeals process. Wait -- hiring an attorney is not so very awful! So long as you "shop" carefully and get one that is designated a disabilities attorney --and ask, and get confirmation -- they are required by law to not charge you more than a percentage of your first check...and nothing, if they are not able to get you approved. So if you are denied at first go-round, hire the attorney. Chances are much better that they'll get you approved than if you try to appeal on your own.
Don't be alarmed if they ask you to see "their" doctor...or even "their" psychiatrist or psychologist. The exam will be probably inconvenient -- they'll schedule a time and place without consulting you, and it often turns out to be far away and at a very inopportune time...but you must go. If you don't, they'll deny your application without further ado.
The visit is usually cursory, brief, and not terribly revealing...but part of the bureaucratic process.
Good luck! Be patient. You'll encounter all sorts of bureaucratic snags along the way, but persevere.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....