Both my boys started on the shield... Brendan fed on the shield for 5.5 months (and needed top-ups afterwards) I had lots of troubles with him, he had a weak suck and I later found out he was tongue-tied which is what caused our issues.

Tristan was fed on the shield until about 1-2 weeks ago, he now only feeds with it at night (I'm lazy... its easier for him to drink while I'm laying down with the shield on )

When it comes down to it though, you are better off feeding with a shield than not BFing at all. So dont feel guilty you are doing wonderfully.
If you are happy to continue using it then why not!