It sounds like your pleasure reaches what is called the plateau phase and then fades away before you peak out at orgasm. A lot of things can cause the problem you describe.
First you want to make sure that your health is all right. Various illnesses, hormone imbalances, low blood sugar, food allergies, infections, certain drugs, alcohol, low testosterone levels, diabetes, and even a yeast overgrowth can cause a lack of sexual feeling or interfere with orgasm. Have a complete check up and make sure that you're all right.
We also should rule out the possibility that unresolved conflict with your boyfriend is getting you sexually down. Let's also make sure that your lack of sexual feeling isn't the result of another emotional issue that is being aroused by the relationship.
If, for example, you are feeling insecure with your boyfriend, afraid that he might reject or abaondon you, afraid that he won't commit to you, or anything else that leaves you feeling insecure, it is possible that your psyche is making you numb so that you will feel less physically and emotionally attached to him, thereby minimizing your potential pain if and when he leaves.
Last but not least, make sure that you aren't stressed out, depressed, or anxious. Any or all of these mood problems can result in sexual dysfunction.
I have given you lots to explore. Let me know if you need further help in figuring out the cause of the problem.