I have been doing datacomms for 25 years. Started on V21 modems, 300 baud if you can beleive that! I have worked on pretty much every type and make of comms kit there is from modems to wireless to ATM to 10G Ethernet, home networking though to data centre cores. I too went through a period where I couldn't quite believe how fast I was leaving behind technology I had sweated over to learn.
But here's the thing. The more you learn and leave behind the better you become at learning. The more you sweat over how to do the same thing on different kit, the more your skills become generalised. Young engineers, when handed a new firewall will say, "I haven't been trained on that, I can only do Acme firewalls." (I personally would term such a person a technician rather than an engineer, these terms are bandied about far too easily)
An old engineer will say, "A firewall is a firewall, they are all designed to achieve the same end, its just all the knobs and dials are in different places." The point is that because your knowledge is generalised, you know what the kit is supposed to achieve, you know all the switches and peddles are there somewhere, you just need to find them.
And to me that makes you the most valuable engineer of all. Because I know that while you might not get the job done as fast as a technician who does Acme firewalls all day long, you WILL get it. And the next firewall I throw at you, and the next. And when a brand new technology comes along that no-one know about, you will be the person best placed to make it work because picking up new things is your real skill, not the fact that you can spout a specific products command line verbatim.
So don't, whatever you do, consider all that experience wasted. It really is not. In the IT industry things change so fast that the real skill is in the learning, not the knowing. And learning new things is like any skill, you get better at it with practice.