What is wrong with me?

So lately all my friends are all single and i am in a relationship. I will always make time to spend time with both but lately me being in a relationship is not the issue. My age is, i do not mind my friends all hanging out without me....wait yes i do that is the point.. i am currently 20 and they are all 21 so i am not able to go out with them. The thing is that it really makes me sad that i am not invited... i only have 2 months until i am of age. i get more sad thinking about my life revolving it, i get this weird anxiety that they all talk about me and are trying to cast me out. i know i may be overthinking a little but it just sucks not being included. i am not the type to express my feelings with them...