Honestly, OP, I until I clocked you were 25 I thought this was a post from someone in extreme old age who was miserably disappointed in ninety years of life.
OK, first thing - as Jessie said, no one looks at a 25 year old, or a 28 year old, and thinks 'barren spinster'. In fact, in my universe, I doubt anyone thinks of anyone, of any age, in such sexist, disparaging terms. I have wonderful, fulfilled single, childfree female friends in their forties and fifties who are certainly not defined in terms of not being married or a parent. You clearly think that those are bad things to be, but that's your own poor self-image speaking, rather than anything anyone at your sister's wedding in three years is likely to think.
The wedding may not be your idea of a wonderfully good time, but there's zero basis for thinking other guests will be writing you off as some kind of loser.
I can't make myself fertile, I can't make a man marry me, I can't make my mh issues go away. I can't make people like me, either.
It's interesting that you're focusing on what you can't do, or seeing life in terms of what you think is down to fate/luck/something other people have to do for/to you. And if you'll excuse me saying so, it's a cop out.
You can certainly work on your MH issues - other posters have made good suggestions. No one is suggesting it will be easy, but with hard work and commitment to therapies etc, it's certainly possible to make real improvements in many MH conditions. I think that is something you need to prioritise over any of your other life goals. Marriage and/or having a child isn't some kind of cure-all for anxiety and agoraphobia - in fact pregnancy and having a new born can be a real flashpoint for MH issues. (I had post-natal psychosis myself, and found maternity leave the most difficult and isolating time in my entire life.)
And many men and women in perfect psychological good health find getting out there and finding a partner in the first place difficult - ditto fertility treatment.
Which is not to discourage you in the slightest - quite the opposite.
What I'm saying is that much of what you're hoping for - the desire to find a significant other, marry them, and have a child - is shared by a lot of people, most of whom haven't found a spouse or had a child by 25. You're sharing this wish with an awful lot of the human race, and a lot of the human race share your current situation of not having found your partner yet.
But it often takes effort to find a marriage partner, and having a better control of your MH issues is going to be key with dealing with the ups and downs of relationships, trying to conceive or fertility treatment or adoption alone or with a partner, and being a parent.
If you want this, you're going to have to work for it. Good luck.