The thing is I am not good at "talking" about issues especially when they are things that bother me and I honestly don't know how to bring this up. Any advice?
Just be direct. It may be difficult the first time to get started, but really it is the right thing to do. Getting this issue resolved is needed for both of you. So next time he avoids sex or turns you down, just calmly and matter-of-factly ask him "What's really going on here? Why do you not want to have sex with me?". Just say it with as little emotion as possible, which should make it easier for him to answer. If you're all upset or sound like you're accusing him of something bad, he will feel defensive.
I would also have several followup responses thought out. He will likely respond one of several predictable ways. He may minimize or brush off what you say, as if he isn't avoiding sex with you. He may shift the blame to you by saying you always approach him when it is a bad time. He may repeat his previous excuses. I'm sure you could think of several other ways he might respond. So be prepared with something for each of those responses of his. Your reply should take the discussion further, not end it.
You might say that his response no longer is valid because it has been far too long. You might say he is avoiding the question. You might say that even if his reason is valid, the situation is not tolerable. You might say that you see things very differently than he does. And then you can follow up either repeating what you opened with ("What's really going on here? Why do you not want to have sex with me?"), or you can go to something else like saying you want to go to marriage counseling.
Do some research to identify a few marriage counselors nearby. You can even call to interview them. Tell them your situation in a few sentences and ask them if they've dealt with this before, and what their approach would probably be. Select the therapist you feel makes the most sense to you.
If all else is equal, I think a male therapist might be the better choice, but it is only one factor in choosing the therapist.
I highly recommend the book to you "When I Say No, I Feel Guilty" by M. Smith. The book has much more to it than the title suggests. There are numerous verbal tools to use when talking with someone who is being difficult. But there are also philosophical ideas in there which I think would benefit you. Your local library may have it, so you can even read it for free.