Your question is concerned about motivation and constancy, right?
Answering the latter briefly: Yes, constancy is important & also the quality of it. This can be compared to every skill really. This is why meditation should be practised also in daily life (which would be sila really).
Moving on with motivation: I believe that a novice needs to start slowly. Once you reap the benefits, i.e., calmness & tranquility, you want to do it more often because of its pleasant nature.
Personally, I use certain "tricks" to make it easier to start meditating:
Depending on your level of motivation set yourself a small goal, say 5 to 10 minutes & try to achieve that. The idea here is thar usually motivation comes by actively doing the very thing you want to do. Setting small goals in your mind will facilitate action.
Oppose in written form and/or in your mind both options, that is, slacking off & meditating. Write for each option the pros and cons, both short-term and long-term. Imagine how farther you could be in, say, a month from now if you'd meditate regularly (both formally and informally). Then, imagine how you'll end up if you slack off/procrastinate. Keep the costs of procrastinating and the short & long term benefits of meditating well in your mind and really focus on them several times a day.
Know your WHY. What is the reason you want to meditate? What's in for you? This is strongly connected to number 2.
If you, for example are usually practising mindfulness with breathing, I'd suggest to maybe change the meditation (at least in the beginning of the meditation)
Discursive meditations can be easier at the beginning than breath awareness because the breath is a rather neutral object, whereas loving-kindness (or any other thinking meditation) is more stimulating because you don't force the unruly & stubborn mind to be quite; instead you let the mind think, but in a disciplined and organized way.
- Realise that conditions must not be the way you want them to, meaning: You can still take actions even if you feel no motivation. This is uncomfortable, but remind yourself that you can stand the uncomfort & that it's worth standing it (because you have a goal in mind).
With that said, there is feeling-based motivation and value-based motivation. The former can be quite strong, but it's dependent on you being motivated. Unfortunately, motivation often comes by doing something. If you can accept this grim reality, you will be better off, and not fall prey to your current frame of mind.