I don't think there's any evidence suggesting that mindfulness or meditation are the opposite of self-regulation. In arguing why, it'll be useful to define terms.
Meditation and mindfulness
First off, meditation and mindfulness are not the same thing. Meditation generally refers to a family of practices for investigating or inducing different states of consciousness. For example, "loving-kindness" meditation requires focusing your attention on the feelings of love and closeness you have towards someone in your life (and has been studied by Fredrickson, et al., 2008). There are many forms of meditation, and so the link between meditation and self-regulation could vary based on the type of meditation one is thinking of. The type of meditation you seem to be thinking of is mindfulness. So let's talk about what mindfulness is.
Mindfulness is generally thought to have two components: 1) a regulation of attention to conscious experience of the present moment and 2) non-judgement or non-evaluation of that experience. (a definition I paraphrased from Greenberg et al. (2012); note that they discuss alternative conceptualizations as well). So, when you catch yourself in yoga class, daydreaming about what you're going to make for dinner tomorrow, and then stressing about whether or not your dinner guests will like it, you are failing both criteria for mindfulness. Conversely, if you find yourself in a yoga class, aware of how each of the positions makes your body feel, and you simply notice (rather than judge or react to) these feelings, then you're being mindful.
Emotional self-regulation
Your question relates to the link (if any) between mindfulness and emotional self-regulation in particular. So, let's clarify what emotional self-regulation is.
Emotional self-regulation is the ability to override one's undesirable emotional impulses into more desirable ones. For instance, you may find your mother-in-law insanely irritating. When she comes around, you have a strong impulse to yell and shout about how obnoxious she is. However, this action doesn't really jive with your goal of maintaining a peaceful, pleasant relationship with your spouse. So, if you manage to suppress the urge to shout at your mother-in-law, and replace it with a more acceptable tight-lipped grin, you've successfully self-regulated (much better than I probably could).
Link between the two
It seems that mindfulness ought to increase one's ability to self-regulate in this way. Specifically, mindfulness should help one A) identify the emotional impulse as it shows up by increasing attention to one's present moment and B) neutralize the intensity of the emotional impulse by reducing the extent to which one justifies or identifies with it.
If you're viewing emotional impulses mindfully (noticing them as they show up, and not judging or evaluating them), then you're simply treating them as perturbations of conscious experience. In other words, they're just another change in your experience, as if someone had changed the color of the lights in the room. Seeing emotional impulses as mere changes in conscious experience creates some mental space between you and them. All of a sudden you don't feel so identified with your emotional experience (any more than you feel identified with the color of the light in the room).
The literature offers some support for these arguments. Brown & Ryan (2003) find a positive correlation between trait mindfulness (one's general propensity for mindfulness) and several constructs related to emotional self-regulation. It's correlational work, so we're not sure which of these is causing the other. Nevertheless, the signs of the correlations suggest the concepts are, at the very least, not opposites. More mindfulness tends to be associated with more emotional self-regulation
References
Brown, K. W., & Ryan, R. M. (2003). The benefits of being present: mindfulness and its role in psychological well-being. Journal of Personality and Social Psychology, 84(4): 822. http://selfdeterminationtheory.org/SDT/documents/2003_BrownRyan.pdf
Fredrickson, B. L., Cohn, M. A., Coffey, K. A., Pek, J., & Finkel, S. M. (2008). Open hearts build lives: positive emotions, induced through loving-kindness meditation, build consequential personal resources. Journal of Personality and Social Psychology, 95(5): 1045. http://psycnet.apa.org/psycinfo/2008-14857-004
Greenberg, J., Reiner, K., & Meiran, N. (2012). “Mind the trap”: mindfulness practice reduces cognitive rigidity. PLoS One, 7(5): e36206.
