You can do it! Practice, practice and more practice. Supporting your quilt by additional tables will work wonders. A long stitch will help as well.

I so agree extra support for the quilt and a longer stitch will probably help alot. Do not give up !!!!!!!!!!!

Yes, try the extra support with an ironing board or table to the left of your machine. The whole quilt should be on a table, none should be hanging over the edge.
When I first got my walking foot, my dealer told me to make sure that you have a little hump of extra fabric in front of the needle. The fabric shouldn't just be flat on the infeed, the little hump or pleat means that you're not pulling on the fabric and it lets the walking foot actually feed properly. As you sew, you keep gently pushing the fabric forward to maintain that little hump, guiding it into the foot. I just used my walking foot to attach a binding, and used this method. Quilt feed very smoothly. Oh, and i always have a quilting glove on my left hand to guide the fabric and move it forward.

When doing any kind of quilting on large quilts, I put my ironing board to the left of my machine, as up close as I can get it....then let it take the burden of the quilt weight....I don't roll my quilt as it seems easier for me to puddle it ....then I can move it out of the way easier.

I set up my ironing board and tv trays around my machine to make sure the quilt is supported well and nothing is hanging down. That helped tremenduously, as it seemed that the overhanging parts were weighing down the portion that I was sewing and pulling it all wonky.

I learned to stitch in the ditch from taking classes. That's all I knew how to do for a long time. I learned to free motion quilt and that's all I do now. SID is really hard I was told by a lot of quilter's. I just didn't like the way it looked. I watched Leah Day for many nights and taught myself.

Have you tried starting out with a smaller project to practice your skills? How about making a set of placemats, or a table runner? You can set your current project aside for now, and make something quick that will help you decide if it's the weight of the large project that is causing problems for you. I agree with the idea above about adding tables to your work area. You can also use a dining room table, and position your machine so you have the most space behind and to the left of your machine.

The length of the stitch depends on your machine, the thread you're using, and size needle you're using. I took a class on "Craftsy" wherein the instructor said to go with the default stitch length. That way you don't have to reset it every time you start up again.

At any rate, it sounds like quilting in sections or starting with smaller projects to gain confidence and experience may help you be less frustrated.

I think it is more difficult to SID than to just do a stippling, cornelli or meander stitch all over the quilt. If you are a beginner, I suggest you give it a go and you will get better results. Do some practice pieces first.

As a handquiter I am always amazed that anyone Stitches in the Ditch. I want my stitches to be visible. The "quilting" is what makes it a quilt. The only time I SITD is if I am making a quilt out of "cheaters cloth". If I hand stitch around each of the motifs most people can't tell it from a pieced quilt. When I "quilt" a pieced quilt I usually echo the piecing lines about 1/4" on each side unless I am doing an overall pattern of flowers or hearts etc. etc.

Just my two cents.