Losing feelings (vent??)

Have you talked to him about this fading feeling? It could be the need for communication. Maybe, as well, try to find things to do with him because it seems that you want that spark back but just don’t know what to do. Plus this happens it is okay to have feelings of uncertainty, but it would be beneficial if you communicate your feelings, find an understanding, and keep moving forward.

That's kind of normal. https://en.wikipedia.org/wiki/Limerence

Don't fall for the trap of "chasing the high" though. A lot of girls fall into that trap and end up used up girls that hate men. It also sews a lot of negativity into the male dating pool as well. Because they are like what the heck I did everything right and she still left.

If there are other things he has done or bad things about the core of his character you have learned about on the other hand, then that would be different though. 

Finally, people use the term "manipulation" a lot, but really what most people are calling manipulation in the last few years isn't what manipulation used to mean. it's just what we used to call communicating needs and wants and doing normal things to attain them. Manipulation is usually malicious, like you are setting someone up in an unscrupulous or unethical way. 
For example, if you don't want your boyfriend to hang out with his friends because you want to spend time with him. If you tell him that you feel like you want to spend more time with him and you would be really happy if he does, and sad if he doesn't that's just communicating and a healthy relationship. On the other hand if you tell his friend's that he secretly hates them and to stop inviting him to hang out. Or you tell your bf that you heard his friends actually hate him and they just make fun of him for hanging out that would be an example manipulation.

maybe you arent losing feelings, just getting more normal with him and the relationship. once you date someone for a while it doesnt feel like how it did at the start. it may seem like you love them less but in reality you dont, youre just comfortable. if you really are losing feelings then try doing activities that you love to do with him and try to bring the spark back. and sometimes people go through phases of feeling different, maybe its just a phase that you will get over and you will love him just the same. id say just wait it out and if you still have lost feelings then its time to let him go.