Devonsmummy started a thread asking for practical advice and support and was frightened off it by no doubt well meaning but overzealous posting.
This is a thread to allow for more gentle support, if you are still reading Devon.
In terms of your immediate problem, improving your credit rating is a very sensible reason, and if he pushes you as to why you didn't tell him, just say that it was at the same bank as you have your joint account. If you wanted to keep it secret, you would surely have opened your account at a different bank, not one where the post may all come in one envelope . . . .
Others will have more advice about your situation, but gently ladies please.