I get extremely anxious as my birthday approaches (especially "important" ones like 16 or 18?

Almost everyone has "anxiety disorder" from time to time. Pleas stop telling yourself that: focus on solutions rather than the problem.

While I appreciate that this is a serious question concerning an issue that is rightly important to you, we older types often wish we had not taken everything and everyone (including ourselves) QUITE so very seriously during our short teenage years, so be cool, use humour and your natural warmth - we all have it, but some are frightened to show it. Adolescence (approx. age 12 - 21) is a period of emotional change and discovery – so much changes! We can’t avoid changing, but we CAN make decisions about which direction we wish to go in - the sort of person we want to become. The important things are to develop self respect and emotional self sufficiency (i.e. not relying too much on someone else for our happiness) and to work on our values and qualities such as integrity and respect for ourselves and others: absolutely everyone has things about them you don’t know about. Also ensure that you have no unnecessary fear (False Evidence Appearing Real!). We CAN decide about and plan our directions by practising in our minds the kind of person we wish and intend to be. Especially first and last thing in the day when the subconscious is most accepting of your conscious thoughts. (So avoid negative t.v., video games, etc., especially at night).

Life tends to come in seven year stages (or maybe six is the new seven!). Infancy, then childhood then adolescence, which is usually around 12 - 21. Naturally people vary, and the stages merge into each other like the seasons do, but it continues to an extent: many women are not too mature until late 20s and a lot of men don’t even start to grow up until their mid thirties (I was one!). Actually Judge Judy said recently their forties!

All parents have been teenagers of course, and can be taken back to their teenage feelings with genuine interest and positivity on your part. That said, no one person can ever completely understand another (except identical twins, perhaps): we all come from slightly different places and have different experiences of and takes on life. Hence nobody can judge us with any accuracy.

Remember: adolescents’ emotions are in a state of flux and constantly changing – some more than others, obviously.

Doing things for others without asking for anything in return, such as voluntary work is very often a key here (what goes around DOES come around eventually). That said, allowing others to do things for us makes them feel good – all in moderation of course!

I've stopped caring about birthdays. It's odd how we celebrate for being one year closer to our deaths. To me, that isn't anything worth celebrating.
It's not weird to invite people from different groups to your birthday. After all, it's YOUR birthday, not theirs. It's about you, not them. If they can't come to a party just because a stranger is there, then they better suck it up. I'm not saying they need to pamper you hand and foot on your birthday but basic respect and choosing what you want to do is acceptable and expected. I've been to friends' birthday parties where my friends invite other friends from other schools/groups that I don't know and I don't mind at all. Sometimes people just want to share their birthdays, an important moment, with people from different parts of their life.