Hi
My wife has been on and off SSRIs for years and has finally accepted that she needs them to keep an even balance in her life. This is going to be a long post because we've been together over 20 years and she's been on and off them all that time!
She has found (and I have observed) that they all behave differently. On some, she had no interest in sex at all and it really reduced the frequency that she would want something for herself. Others were a bit different in that she'd start out interested, and in fact would get excited, but then she would have really dificulty getting over the line. This was worse if she was tired or stressed, so again the amount that she wanted sex tended to go down as getting to the orgasm was hard and not guaranteed to work.
Now, she's got older and more confident, she kept going back to the doctor. She didn't even really have to explain, she just said that the pills were helping, but she wasn't happy with the side-effects and they gave her a different set. By experimenting like this, and working with her doctor on the dose, she found one that seems to work to improve her mood but also allow us a healthy sex life. We also quite often use something like Zestra or Durex O gel to help things along and she nearly always uses a vibrator.
But, to answer your original question, IMHO (and I'm not medically trained) you haven't been significantly desensitised by the vibrator. Different pills, even if they're meant to work the same way, can have a very different effect on sex and the ability to reach orgasm. And the change in effect can be very quick indeed. To the extent that if we know that sex is on the cards (we have three kids so some planning is nearly always required) then she sometimes holds off on her antidepressant until afterwards. Usually this is if she has the day off and all three children are in school. So, instead of taking her pills first thing, she'll take them lunch-time. But even that effect of it being 26 or so hours after the last one can be enough to help.
But what you're saying sounds typical - one set of pills, really hard work with a vibrator, for ages, to come. Change the pills and you're there really easily with your fingers. I would say, really, really strongly, work with your doctor. See if you can find a set of pills that keeps your mood stable and allows you a reasonable sex life.
Hope this helps, and do get in touch or ask questions if you need to!