I need help

You essentially need a procedure to determine when the entity is blocked, and what block to mine. I haven't done something like this specifically, but I think I've got some general pointers for both problems.

• There's various ways to detect when an entity is stuck. I did something sort of similar for termites; if their x or z velocity is zero for long enough, they gain a status effect that tells them to change their behavior. (If you wanted to do this more effectively, you would need to both check for lack of movement in either direction, and check that there are solid blocks between the entity and the player, which would require calculating some vectors.) 
• For most cases though, just detecting if velocity is at or near zero will tell you if it's stuck. You can increment an NBT tag or give the entity increasing duration of a status effect when it isn't moving, and then if the tag/status effect gets high enough, add another NBT tag or status effect that tells it to start breaking blocks.
• The block breaking part shouldn't be too difficult, you just need to target blocks correctly, and possibly make a tag for which blocks it's allowed to break.
• To actually target the block, use the 'look at x/y/z' function to make the entity look at the player's positon, then get the block at the entity's x/y/z, plus the x/y/z look angle vectors of the entity. These are vectors that are based on the direction the entity is looking; if it's looking east, the x vector is 1; if it's looking west, the x look angle vector is -1. North or South yields an x vector of zero; and looking southeast yields an x vector of 0.5. This will essentially give you the offset that the desired blocks are at. (You might want to multiply the vectors by something like 1.5, just to make sure it hits the correct blocks.)
• If you want the entity to break blocks gradually, it's a bit more complicated, but not impossible. I assume though that you want it to break stuff pretty fast. Even still, might wanna give it a brief delay.

Also, good job being descriptive and including your version type, a lot of help requests can be frustratingly vague! Maybe title stuff a bit more descriptively though, otherwise mods aren't super likely to help. =)