I am a Grammy that doesn't get to see her grandson and I am very resentful?

My son met the baby mama in high school. She broke up with him 2 times before they got back together the 3rd time and she got pregnant. They stayed together during the pregnancy and a year after B was born. Baby Moma will be called K. I was there at the hospital when B was born and her family totally took over and I barely got anytime with B..so I figured there will be plenty of time to see B so I went about my life. She used to bring him to see me once every 2 to 3 months. I told her I didn't mind watching B while she has errands to run or anytime I would love to spend some time with him and she says she's not comfortable leaving B at this time that she can't be away from him. Me being a mother myself I do understand this. I did not gripe about it, I was very supportive. after B's 1st birthday I again told her I would love for B to come over or stay the night or just for a couple of hours. She still said she's not comfortable. She felt guilty and started bringing him to my house with her guidance about once every 2 weeks for 6 months. I was ok with that. Now his 3rd birthday is coming up and I asked her if I could come get B sometime. and she still says Uh no. I asked why and she says until B comes around you more and is comfortable with you. Easter I dropped off his easter basket to her parents because that is where she lives and B was there with her whole family but She was not there, I felt so resentful at her whole family. I am sick and tired of this. He will be 3 and I am very hurt. His 3rd B-day is coming in July and I don't want to go. I told my son I did not want to go to the party because I may have a bad attitude watching her whole family be cheerful and take him over all the time like they did at the hospital. She texts me that this is not about me it's about B and I do understand it but I am only human and don't want to ruin B's party. My son won't exercise his visitation rights cause they have apparently gotten back together and working things out. So what I am asking is: Should I make myself go to the party or not. and how do you deal with this type of family whom obviously has no respect for anyone else.