Problems:
-Space Stock. It is where 99% of people start photo manips but trust me it is not where you want to start one ever, and normally stay away from swapping the sky for a planetary stock, especially when all you do is change the blend mode on it. When you swap a sky use a soft edged brush to clean up the edging along the horizon to keep it neat and fit.

-The Text. Another noob mistake for sure, almost all photomanips should not have text unless your extremely good with 3D modeling which when it comes to C4D or 3DS Max your either amazing or mediocre and i know only few people that are amazing. If you insist on putting it in render it at the same angle the photo was taken so the planes match accordingly.Watch the shading on it aw well, if you look at any of the items in the original there is no angled shadow so the sun (light source) is most likely at a 12 o' clock median, so the shading will be vertical.

-The Pool. The 1st thing you notice about the pool of water is the angle, when searching for stocks you want to find one at a similar angle and then use perspective warp to finish off a perfect angle. The 2nd thing is the color on it does not match your image so you can clearly see it stand out. My last thing about the pool is placement and blending, i would look into a soft brush with a low opacity on the mask and singly brush in the blades of grass to get the best effect, and the placement is a bit odd.

-Color Correction. As most new kids do they don't know how to make realistic color correction that keeps tones fit and does not over saturate or over expose which 90% of people that use CC on this site do. DO for that i made a tutorial on how to make basic realistic color correction which i will sown at the end of this post.

-Finishing Touches. Your cinematic bars are a bit too thick for this but i personally don't like them at all when it comes to manips its kind of a nooby thing to do. And your tag is in a very strange place and try to stay far away from strokes until you know how to use them.