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Currently attempting to convert some standard tactical marines into sternguard to save money, not working out how I want them to (tell me if I am wrong) but I think it looks like crap. How should I improve?

I'm in the process of doing the same thing...mostly because i despise (with a passion) metal minis (coming from the guy with a full up SoB army!!!).
I plan on making them different enough that you can tell them apart at a glance. A simple way would be to just paint the helmet a different color than the rest. In the case of your color scheme maybe go with red helemts.
I also plan to add the terminator crux to the right shouldpad as a squad marker.
Other people have used the "Dark Angels hooded helmet."
The important thing is to make it pretty obvious the unit is NOT a tactical squad at a glance.

Maybe its the robes. They seem a bit too much. You could try just doing long loin clothes or hanging "parchments" on the shoulder pads, use the Mk VI helmets and studded shoulder pads etc.

The robes are so big and cover so much that if they arent done perfectly, lord knows I cant do them, they look like a big blob.

try buying the dark angels upgrade thats what i do if i want robed plastic marines

The back of the robes look pretty good to me. I think you need more form and definition in the front, especially the hood. Maybe work slower and do less at a time? Something else you might consider is using some of the plain crested shoulderpads from CSM boxes to get that "Older thus fancier" look for your armor. I think chapterhouse makes a shouldpad similar to them.

The robes lack detail. Note that simply draping a sheet of Green stuff over the model wont work, as the material will not bend like a real cotton robe, especially since the model is way smaller than real life (thus, the cotton robe in scale to him would be microscopically thin, creating even more folds). You should also re-align the backpack, as it feels like it's on an angle. I know the marine is leaning, but I think the backpack is leaning too much. Align it with the torso first, glue it, then do the dynamic posing.

If you want some easy robes, get the DA upgrade pack. It has robed heads and legs, costs roughly the same as a tactical squad, and comes with a host of other bitz you can use. For Sternguards, note that they tend to have really old armors or other details (most notably studs , collars, ancient helmet patterns, and the likes. There are a few tutorials on how to make these, as well as the DA upgrade sprue coming with a number of these options as well).

Look up... if I got the name right... "what what's greenstuff tips and tricks"? It's in the tutorial section and has some great step by step advice on doing robes and rope.

The back pack is off kilter, the robe lacks definition and the hood looks like hair. If you look at DA SM you can see that hooded helmets have the hood covering to just above the eye, and around where the mouth would be. If you fix the back pack, fold the robe more, and bring the hood around his face further, he would probably look pretty decent.

Keep in mind that the cloth of a robe like that is not a triangular piece of fabric. It is a rectangle gathered at the top. This is what creates folds, as the top is scrunched whereas the bottom, hanging free, opens closer to it's full length.

Your current robe would make a fine base, but try to add some GS on top of it to make folds that get tinier as they near the top. Check out the pictures or Vanguard Vets without jump packs on the GW website to get ideas for how folds should look.