am i suffering from depression? what can i do to stop it?

so its the end of january and i live in the northwest so its always cloudy and rainy and gray. but anyways ive always had this weird thing where i get a little sad in the winter time and when its summer im always very springy and generally more healthy both mentally and physically. ive always been able to deal with this and get through it. it never interfered with my life. but this year is very diffrent. i lost all of my friends and boyfriend. i dont know why it happened but everybody just pretty much turned to rugs and i guess didnt realy value the relationship. usually i am surrounded with people ive always been upbeat and able to make new friends all the time. but for some reason its an akward challenge im litterally all alone i dont have 1 person in my life. i try my very hardest to keep a good mind frame but then for the past few days at the same time evry night i get extremly depressed. at times i cant sleep and its hard to be my normal self naturally. but then at times im okay and im not depressed. what should i do? im not suicidle at all or into hurting myself but i feel like if my dog were to die tomorrow or anything like at that level, i wont be able to handle it. ive been very bored and anxious about after highschool since im graduating this year. and im very lonely and lost hope in people. is this depression??