Daniela,
In your post you wrote, "...hard for me to live my life with the outlook that I may have to stay single." To me you are still young. My nephew got married last Sept. at the age of 38 & his wife is about the same age -- first marriage for both (however, neither are believers - but still they both married at a later age).
God's Word is so comforting & our Heavenly Father gives us so much wise counsel. In Matthew He tells us "to not be anxious for tomorrow... each day has enough trouble of its own" Young gal, none of us know what the future holds. God doesn't want us to speculate what may or may not happen. He wants us just to think about today because that's all we can really handle. For anyone it is overwhelming to wonder what might happen down the road. He doesn't want us to handle our entire life all at once.
"Don't be anxious about tomorrow" -- God calls us to live each day one at a time. He's not asking you to look down through life imagining you'll be single FOREVER. Yes, you are single for today, but... God's plans for you are good. Nothing will ever happen to you that He doesn't already know about. Nothing that ever happens to you is a mistake. God will use everything that happens to you for His glory -- He will use it for His good. Don't let the thoughts of "what if" rob you from the peace & joy He wants to give you for today. I too thought I wouldn't be single at MY age. The younger years were harder, but now... some days it's hard... if I allow myself to think about things God has told me not to think about (usually lies from the enemy or my self-pity). Now I can't believe how quickly life goes by -- where has all this time gone? What have I done with my life as a Christian single woman? If I were married I'd ask, "What have I done with my life as a Christian married woman?" What have I done for the Lord?
I can get negative with my thinking and start complaining to the Lord. "What will happen if I can't take care of myself when I'm older? I have no one to take care of me." "I'll have to work forever because I can't afford to live on the retirement I'll get. Others have their spouse or kids or grandkids to help them, I have nobody." "My job is getting harder. I've never done anything else, what will I do?" In my mind I'm thinking if I had gotten married & had children somehow I would not have any of those problems. Perhaps I wouldn't have the things I worry about, but there'd be other things.
Singles ask,"why aren't I married?" Married people ask, "Why did I ever marry him (or her)?" "I wish I hadn't." Whether we are single or married have children or are childless God oversees every detail of our lives. He is using those things to "complete the good work in us."
I have a friend that had a baby last year. They had tried for a long time to get pregnant. Her pregnancy was very difficult. At five months she was admitted to hospital & put on bed rest because she was starting to go into labor & the drs. were trying to stop it. Things went well for a short time, but she delivered him at six months. He was in the hospital for several months before he could be brought home. He has had a lot of problems. He is not developing like other children. When you see him you can tell he has special needs. He is year now & still not crawling or walking... When I think of the problems I think I have -- when I look down 40 more years of singleness I can begin to feel sorry for myself. But when I think of my friend I think how difficult things must be for her & her husband. I think of how they have to control their thoughts & not worry about what will happen to him in 20 yrs. or they won't be able to meet his needs for today. Then I always think, perhaps by me being single God is protecting me from some type of other heartache that would be more unbearable for me. But the Lord gave this little boy the parents he needed that will love him unconditionally & will be the apple of their eye.
Keep on loving Jesus & remembering He has your best interest at heart. Yes, the Word talks about God giving us the desires of our heart. But since He is God and knows all things... but I think sometimes what we desire may not be best for us. Couples desire children, yet remain childless. People who are paralyzed desire to walk, yet God does not heal them. God has His reasons. Daily as we draw closer to Him sometimes those desires change. Just keep remembering how much He loves you, His plans for you are good, all things work together for His good, He will complete the good work in you -- molding you into the image of Jesus, and you are the apple of His eye. These things are true & God tells us in Philippians to think about those things that are true & lovely & to not worry about anything.
Some things this little, old lady has learned

! Being 51 AND ssstttillll NOT married isn't the worst thing that can happen in one's life

! God is good ALL the time.