I´m not sure if every one of your stories fit in the same scenario, but I hope this helps:
When a man who loves doesn´t "feel like" having sex with you, there sure is a problem, but it´s rarely ever related to sex.
Men like sex, and they enjoy it, but it is true that they sometimes feel pressure, and are not able to perform at their best, which makes them even more frustrated, and the more frustrated you are, the more pressure he feels, so first of, stop "asking" for sex, even more so, stop demanding it if you are. Be sublte. Men are not very attracted to a woman pursuing them (they are when they can´t get any, of course, but if he loves you, and he already wanted you before, you don´t need to offer it, I know it may be difficult, but restrain yourself from going after him. It´s okay to tell him he looks sexy, or that he is handsome, but stop there, he has to pursue you).
Second, this is a very common cause of men not having sex with the woman they love, they feel pressure because they feel they are the only thing on your life, or at least, your main focus. If you´re not too happy on your job, or you have stopped doing your hobbies, he will feel like all of your happiness depends on him (most men are egocentric like that, this is not a bad thing, it´s just the way they are wired). Go back to your old hobbies, or find a new one. Find something to do that you enjoy and that does not depend on him at all. Tell him about it, and about how excited you are about this new thing that you are trying (Try to make it something that he won´t want to join into, this needs to be something you do alone for yourself) but don´t fake it. Do something that you´re really excited about. Avoid things like cooking, or projects where the end result will still benefit him (If you take on knitting, and knit him a sweater, he may feel like it still bounces back to him, if you enjoy knitting, then knit things for yourself. Focus on you on this one).
Keep taking care of yourself (shallowly speaking, don´t give up on that) and flirt with him. Be subtle and discreet. Place your cand on his, and maybe caress him gently with one of your fingers, stare at his lips, lick yours, but do not engage in physical action until he does. Do respond if he does, but always give less than him, if he kisses you, kiss him back, but don´t throw yourself at him after one kiss. Draw back. Let him pursue you.
Like I said, this won´t fix all of your problems, you better than me know if he might be cheating, or just straying away. This response is focused for when he used to be sexual with you, your relationship is emotionally commited, you know he loves you and he just feels too pressured to "perform" in the bedroom with you.
Focus on yourself.
Set goals for yourself that have nothing to do with him, and focus on them.
If the problem is really just the pressure, this should fix it.
Men are wired differently than we are. They are less aware of their emotions and where they come from, if he says he´s feeling pressured, he must be feeling it a lot for him to notice, and accept it, they are just not always usually aware of the fact that the feeling of pressure comes from the fact that they feel like you have all of your hopes set on him. The more you want it, the more impact he thinks the disappointment will be.
I hope this helps you. A good relationship is not worth throwing away over sex.