If your using a walking foot, & its in good working order, all you need to do is be sure your keeping your fabric. straight The walking foot, machine, and feeddogs should carry the material through evenly. May have to check on the stitch length and tension however. Don't give up like others have said it just takes patience, trying things, practice, and time. You'll get there.

i guess I didn't read the original question like others did and I didn't read all of the answers. So.....
Inlwhitt said she is using the walking foot. So therefore stitch length has nothing to do with pushing the work thru or speed of the motor.....So lets start over. However, there is a sentence after walking foot that puzzels me. Are you doing both walking foot and some free motion. This is confusing.

If only walking foot - I suggest you lengthen the stitch slightly to accommodate the extra layers of fabric. My machine is set on 2.5 so I lengthen it to about a 3. Fee dogs up and hold the fabric so there is no drag.
Should give a consistent stitch.

If you are using the walking foot - you are not responsible for the length of the stitch. The feed dogs do that.

Also you said "this week we are supposed to do th quilting.......etc" Should you be doing this in class or was it homework. If homework then I would assume the instructor gave some information about how to do it.
If not homework
then perhaps should wait for the class for instruction.

Decorative stitches can be problematic for anyone. I think the problems (for me at least) occur when the stitches build up a little bit too thick and then the fabric doesn't feed evenly. The feed dogs are pulling the fabric through, and your hands should only be guiding it gently. You might want to experiment with changing the stitch length and/or stitch width on your machine until you are satisfied that the decorative stitch will work with your quilt sandwich. Practice on a quilt sandwich using the same batting and similar fabrics until you have it adjusted to your satisfaction. Some of the advice you've been given is excellent advice, but meant for FMQ (free motion quilting), not for your current situation. The speed you're slowing at doesn't affect the length of the stitches when you're using the walking foot (feed dogs engaged) and you can't do decorative stitches when you're doing FMQ.

Thanks so much. This has been happening to me also. I appreciate you help. I believe that you have hit the nail on the head with this answer.

I've noticed that stitching decorative stitches over intersections where seams meet changes the size and shape of the stitch. My solution was to use the more simple stitches and to lengthen the stitch a little. Also--don't look too closely at your stitches--we tend to be overly critical of every little error. I know you want to do your best and learn correctly and that is good but I don't think perfect is a goal many of us achieve every time. Sometimes we have to accept and just keep working at getting better. So glad you are learning to quilt, it is a great trip even with a few bumps in the road.

I lengthen my stitch to 3 and 3.5+ if the batting is extra thick when using a walking foot.
As said in previous answers, make sure your quilt sandwich is laying nicely so it feeds through evenly. You may have to start and stop a lot to adjust it, but the results will be much better :)
If the decorative stitch you are using is intricate, it can give you problems. I would suggest the Serpentine stitch, or one that is very open and simple. Change this stitch too, longer and maybe a little wider also, but check that it is not too wide for your foot :)
As you get more experience, you can try out more complex stitches :) :) :)

Like many have said I think the decorative stitch you are using may be a problem. Choose a very simple one.

I think with the walking foot it is made to walk forward best. Any fancy stitch that reverses to make a pattern may be problematic?

I do have the feed dogs up and will try again tomorrow. I sure hope it goes better. sigh...Thanks for all the advice it helps alot. Atleast I feel up to trying again. I just want to be ready for our last class on Wed but if I'm not I guess I'll deal with it.

I am not doing free motion quilting (yet) as someone asked just the quilting along the seam line using a decorative stitch. We were given instruction and practiced on our practice sandwich the week before and I did okay then. I think I just must not have had the quilt and all the weight correct on the table and machine so it was dragging. Hoping tomorrow is better. Thanks for all the tips and suggestions.