Pearson (the awarding body for BTEC) don't mandate how colleges split-up the two-year BTEC Nationals Level 3 Extended Diploma between the two years. However, logically colleges will do two of the external (examined) units in first year, and two of them in the second year. The examined units aren't all the same size (in terms of GLH), so don't receive the same points.
Let's assume your college did units 2 and 3 last year, and are doing units 6 and 7 this year. Passes in units 2 and 3 last year would have earned you between 21 and 33 points (depending on the exact mark you got). As you say that "last year I didn’t really attend College or put effort in", you're likely at the lower end of that range.
You'll have also likely taken 1 or 2 mandatory and about 3 optional internal (coursework) units last year. Each mandatory internal unit is 90 GLH, and so would have earned you 9 points for a pass. Each optional internal unit is 60 GLH, and so would have earned you 6 points for a pass.
So in total from last year, you'll likely have earned about 21 + (2 x 9) + (3 x 6) = 57 points. For a DMM you need 176 points in total. Each distinction in a mandatory internal unit is worth 24 points (but you've likely only one of those still to do), and each distinction in an optional internal unit (of which you'll likely have three to do) will earn you 16 points. So you're probably at about 57 + 24 + (3 x 16) = 129 points, with just the mandatory external (examined) units to go. You're 47 points short of a DMM, so you need about 23 or 24 points from each of those two units. You'd get that from a low-ish merit in each one. Is that plausible?
Below if a mock-up of how this might look. The cells in green are last year and the cells in blue are this year. Obviously I don't know what units you've done or what units you've still to do. However, you can see that it's
possible to get DMM overall.
For a more accurate prediction you need to be much more detailed with the information you provide. I'd need to know the unit number of each unit you've taken (and the grade, or mark if you know it for external units) and the predictions for each you haven't yet taken.