It will get better with time. We have two indoor cats, we got one, then another 8 months later.. both female.
Both my cats are VERY attached to us, being indoor cats they are our best friends, more like little dogs.
When we got the second cat, our first cat was not happy, at all! litter tray problems, growling and hissing, even growling at us!
We did it the wrong way really, and forced them into each other's company (while watching) and after about 3 weeks they were fine. They are now best friends and do everything together :)
Apparently I should have left kitten in the cage, let the existing cat have a sniff and a hiss, and then leave the kitten in a separate room/area where they can smell each other and but not have contact, then gradually introduce.
Our full steam ahead approach worked though ;)
Cat's are funny onions. I'm sure in time they will be ok.