I've been up against this wall and back again for longer than most of you (about thirty years now), and I agree with most of what has been said.
The way I see it, you can make a decision to learn in your head easily enough - the real trick is to convince your _body_ that all this pain is worth it. You have to convince your whole body that you _need_ to learn the language. Wanting isn't enough. That is why external stimuli like immersion courses work. You get dropped into a situation (as young children do) where you "need" to use the language.
For me, Irish culture offers a myriad of other ways as well to stimulate that feeling of need. Whenever I have hit the wall, I only have to put on one of my favourite CDs with a song I would love to learn to understand one day and the feeling of need is rekindled.
The more of these types of connections to Irish culture you can become immersed in, whether it be Irish songs, poetry, music, dancing, literature, or what have you, the more your body is stimulated by that feeling of needing to learn the language.
Languages aren't just words, they have cultural connections that are integral to them. I don't think it is possible to really learn a language unless you are also really interested in the culture of the country as well.
Over the years I've gotten stuck on certain text books and what I generally find works for me is to work from several books, sometimes simultaneously sometimes consecutively, and definitely not get too hung up about finishing one text before starting another. Often it has been the case that the answer to what got me stuck in one book is better explained in another book so that by the time I come back to the one I got stcuk on, I can see the answer and move on another step or two.
Sometimes it feels like you are going in circles, but as long as you keep at it you start to realise that it is actually a spiral moving steadily forward.
I don't think we ever actually forget anything we have learned - we are just better at finding the stuff we need to know and the rest gets bumped down the chain. I can attest to this because my French gets neglected for years but comes back pretty quickly whenever I encounter a French person. Hence the wisdom of Roberts insistence on being constant.
If you can't afford all those hours Robert mentions though you can still make progress with a little less investment of time - you just have to realise that your progress will be exponentially slower.
If I can't put in hours studying, I make a point of at least reading a little Irish before I go to bed at night - even just five minutes or so. At the moment I am reading a text that is below my level, just for reinforcement. It keeps the language "at the top of my memory heap".
Of course, if you can spend a few weeks or a month or so in the Gaeltacht, all that stimulus is there naturally and it is so much easier than trying to learn by yourself from afar.

In summary:
1. It is no good just learning a language academically (if you want to be able to use it, that is), you have to "live" the language, i.e., attach it to as many real life experiences as you can.
2. The further you are away from the "source", the more effort you have to make to create those experiences, i.e., hook up with other speakers, find classes, go to special events. Before the internet, and to this day, organisations like Conradh na Gaeilge, Comhaltas Ceolteóirí Éireann, and Na Píobairí Uilleann have been creating opportunities for people interested in the language. They are good starting places for finding other speakers in your area.
3. "Keep you hand in", i.e., keep the language at the front of your mind by listening to Irish songs or reading light material, like graphic novels, etc., and talking to yourself in the language. When you get busy and can't do full study, this stops the language being relegated to the dark recesses of your internal filing system.