Find someone to talk to about how you feel. You aren’t alone in feeling the way you feel or in your story
My mother had me at almost 19, after forgoing college to marry my “father”. He abandoned her when I was 3 or 4 months in an apartment after failing to pay for the heat or rent. She moved home and got a job. I was so so lucky this happened and frankly my grandparents, later told me how lucky they felt to have me there. When she remarried I made the decision to stay with my grandparents.
Don’t assume you are a burden on your parents or they don’t want you there. They may feel this way or they may be happy to have you and your son. Why would you be a burden? Don’t you work? Help around the house?
What you can’t do is let this feeling stand and not do anything. Your son will sense it and that is wrong. He is the innocent one here and like it or not you have an obligation to at least make sure he never knows you resent being a mom.
My mom later told me it broke her heart not to take me with her when she remarried but she knew it was best for me. I never knew that as she hid it and supported me in my decision.
Talk to someone and definitely talk to your parents.