Theories of motivation only get you so far when you are trying to maintain a climbing training programme. They describe how and why you might get into the sport and what might be the push and pull factors for you, but if you need help in sticking to a programme, then behavioural theory I find is much more helpful.
Behavioural theory focuses much more on environmental triggers for behaviour and responses to behaviour - an easy way to understand the impact of external factors on ourselves is to think of supermarket layout and advertising. You may start with the intention to only go in for milk and bread, but they are masters at leading you round the shop and capturing your attention so before you know it, you have spent £30 (and you'll likely do the same next time!). Will power is nothing in the face of behavioural nudges and the rewarding sensation of a chocolate brownie or a bargain which caught your eye!
The good news is that you can use these principles to maintain your ability to stick to a training plan. Its all about the environmental cues, the rewards, and also to an extent, the punishers. Environmental cues include things like, keeping your climbing kit by the door or in the car; making going to the wall part of your normal routine (I go on the way home from work so I don't make a special trip to the wall); maybe making sure your route home goes past the climbing wall (alter it if necessary!); and setting an alert on your phone to remind you to go or perhaps taking it in turns to remind your friend to meet up at the wall. Just sticking to the same day every week can also be helpful.
Rewards and punishers are slightly more invidividualised (think tesco clubcard points and the offers you get through the post - they are targetted to what you normally buy so individual to you). Rewards are things which will are likely to increase the behaviour (for example, having a nice coffee at the end of the session), and punishers are things which make you decrease the behaviour (eg having a rubbish session where you couldn't climb something you thought you should have been able to climb and kicking yourself). Rewards are more effective than punishers, but punishers can be more complex and often more subtle and so harder to eliminate (in the example above, having a bad session is a punisher but its hard to tell whether you are going to have a bad session or not, so its tricky to eliminate unless you are really tuned into your mood and fatigue levels). Work out what you enjoy about climbing and in life in general - are you sociable (rope in a partner to your training session and go to the pub afterwards), a foodie (edible treats work well in dog training, why not in climbing training!), a planner (draw up a schedule with military precision), a saver (put a fiver away every time you go climbing towards a short trip or a sports massage etc), competitive (sign up for a competition or the local winter league). Tailor your rewards programme to your own likes and dislikes and personality.
So ways I have built in rewards and punishers to my schedule include, having a nice bath after every session (bubbles, candles, the works); making myself a star chart and giving myself a star after each training session (childish I know but I enjoy seeing the stars stack up!); tracking my progress in a log book (more of a longer term reward, but I can see the progress) and having a snack before I go to the wall as I know this will make me less likely to want to go straight home. In terms of punishers, I have bought myself a 6 month pass and worked out I need to go at least twice a week to make it worthwhile and not being one to waste money, that is motivating me to go at least twice a week! I'm also being mindful of my aches and pains and trying to avoid injury, so adjusting my sessions as needs be so I don't have the experience of having a really bad session. I also made the decision not to get onto the lead wall for a month until I had built up some stamina, since I had been mostly bouldering for the last two years. Instead, I have been working on longer boulder problems (I find bouldering more rewarding than leading usually) and some 4 x 4's and pyramids on the autobelay. I really think this has paid off as a strategy, as I haven't had the depressing experience of having to rest 3 times on a route which I know I can climb grade wise but don't have the stamina to do so. My stamina seems to be improving rapidly and so I am now itching to try it out in November on some (short) lead routes.
Which brings me to my final point - you should be 'itching' to do things and to train. Ok, we all have off days or weeks, but if you are having to force yourself consistently then that is a signal that something is wrong. Perhaps you are a bit burnt out from too much training? Perhaps the goal you have set yourself isn't really what you want after all? Motivation ebbs and flows; trust yourself that it will return if you create the right conditions by perhaps having a break, or just rethinking what you want to train for.