Persisting aggression of old cat to new cat.

Question
Dear Melissa,
I know this is an old problem and I have read a number of previous replies to similar problems but would be really grateful for advice on my specific situation.
I had female two cats for 8 years who got on very well, sleeping and grooming together, etc. The oldest (dominant) girl died three months ago and, concerned that the remaing cat, Tasha, would miss a companion, I adopted a cat, Myrtle, from a rescue centre. Up to this point, Tasha had always been a loving and submissive member of the household. Although I was prepared for some teething troubles at the introduction stage, I really thought she was a cat who wanted feline company. Myrtle, to the best of my information,came originally from a breeder, via someone else to the rescue centre. Her paperwork says she is two years old but my guess is that she is significantly older. She is very fearful and very submissive and I thought, at first, that this would help with introductions as Myrtle would not challenge Tasha for the dominant position.
I set Myrtle up in the spare room with her own litter tray, bowls, etc.
Initial introductions did not go well and involved Tasha attacking Myrtle. So I have continued to keep them separate. Over the last two months, I have installed a baby gate to the spare room, so that they can get used to seeing eachother, I have a dog cage in the sitting room so Myrtle can spend time in the sitting room and Tasha can get used to her in there, Mrtle has the run of half the house at night, I have put a blanket, well-used by Myrtle, over Tahsa's favourite sleeping spot, I have a pheromone diffuser in the spare room and make extensive use of cat-nip, I spend time with them daily, using treats and fuss and play, I feed them either side of the baby gate and am gradually moving the bowls closer to eachother. Tasha no longer flings herself, yowling, at the cage or the gate and she will share treats on the spare bed within a foot of Myrtle without going for her, provided I am in very close attendance, which is a huge improvement, but I still cannot leave them unsupervised for a second with no barrier between them without Tasha seriously attacking Myrtle. This is a totally one-sided affair; Myrtle just tries to escape and hide and I now fear that this is just reinforcing Tasha's behaviour.
I am begining to despair of this ever becoming a working situation and cannot condemn Myrtle to life in the spare room for ever. Please can you advise? Thank-you.

Answer
One thing to know about cats is that they are always going to be territorial with new cats. You are doing the right thing with moving their food bowls closer together. At this point, my best advice would be something I would do. If both cats are willing to be out and about, go ahead and let them. I believe it is best sometimes to just let me fight it out, however have a water bottle ready to use on the attacker, which sounds like it will be Tasha. What has always worked best with me is have some mouth-watering, irresistible food (and possibly use a dish for each if needed) and see if they will both eat. Try a large plate so they don't have to touch each other right away. And don't be afraid that Tasha will hate you if you use the water bottle on her. She'll get over it. Call it tough love. Keep me updated and let me know if you need anything else.