how do you handle missing out

unfortunately i have had a difficult life and i missed out on many life happenings, missed out on relationships, finding a girlfriend, that's mainly it really- missed out on having a relationship...because i simply had a lot of problems, mainly mental health problems..and i'm telling you i've had a very hard life.....now i have lived alone for 18 years in a little flat, living a lonely existence..i'm now past my mid 40s.

how can i handle this and come to terms with the way my life has turned out? missing out on my dreams of having relationships and having fun.