First, all of the fear you are expressing is not unique among converts. Cradle Catholics get that too.
Second, do not fear “losing Catholic faith”. Formally, you can do anything at all and you are still Catholic – maybe not a “model” Catholic for those who notice and judge, but still Catholic. You can renounce your faith or be excommunicated, and you are still Catholic because those things can be remedied without another baptism. One baptism, or acceptance after valid baptism elsewhere, and you’re in until your death.
Third, I consider doubt as an aspect of faith, not an enemy of faith. There are times when I think, “if there isn’t a God, then somebody must have orchestrated that in order to have worked out the way it did.” Like the Holy Spirit had this whole plan arranged and I just fell right into it. Other times it just makes no sense. So I was like, well gosh can’t I make up my mind, am I in or out?
It was clear that especially in the context of “bipolar disorder,” I didn’t need ups and downs caused by my faith in God fluctuating and causing yet another source of stress.
So I took an engineering problem-solving approach.
Given: try as I might to be consistent, some days I believe and other days I doubt.
Goal: find a way to live my life so that I have peace, hope but not ever “expecting” my faith fluctuations to stop.
Strategy: stick closely to what God told us to do about Jesus “listen to Him” and see what that says about how to live. Jesus said “believe in Me” but He didn’t exactly say “you must believe everything your faith teaches you in order to enter the kingdom.” But leaving that reference aside, Jesus’s teachings can bring you into a “kingdom” mentality without even getting into lots of dogma.
Like for example “do not judge” – or that “group identification” is not impressive to God – or that in the kingdom, the one who serves is the greatest. Things like that do not require any belief in dogma, or for that matter even whether He was God. All you have to believe is that there is benefit to reading this. Then trust the Holy Spirit will take it from there.
Focus on what Jesus says, and consider all the implications. Don’t worry about whether you are “right” necessarily, you can use some ideas to explore; it’s not sinful to do that. In fact, in an ancient monastic prayer form “Lectio Divina” (divine reading) the focus is not on getting the “right” understanding, but exploring your own reactions to Scripture and in some groups sharing with others and other group they discourage that.
Also remember Jesus spoke “with authority” in the temple, which amazed people because they’d never heard of it. “Authority” didn’t used to mean citing sources like it does now or wears a certain title (which it could have been but He didn’t – they noted that specifically “isn’t this the son of a carpenter”); it meant He was the “author” of His own statements. Unlike the others, when He read scripture, He took it personally and asked honest questions and formed honest conclusions. In His pure heart, this of course was “authoritative” in that He was God, but what they really saw was a man “speaking His own mind” on issue of scripture. They may have wondered whether he was mad.
So I encourage you to examine your personal ideas about faith, and don’t be afraid to doubt. To me, refusing to allow myself to doubt only would make it stronger because if I have to keep talking myself into something to believe it, or if I know I am hiding doubt, then I’m only fooling myself. If you doubt at the wrong moment, you could sink into the sea like Peter – but then again Peter may have gotten wet but that was an important stage in his spiritual development. Doing the “miraculous” requires strong faith, but doubt is never far away.
Btw my brother’s confirmation name was Thomas. I think Thomas gets a bum rap because Jesus says “blessed are they who see but don’t believe” and I think a lot of people conclude that He means “so they are better off than you” – but I don’t get that from it. Jesus doesn’t generally compare people unfavorably with others unless they are judgmental or haughty – like comparing the tax collector with the pharisee who thanked God for not making him like the tax collector. Plus there was no indication that Jesus was annoyed but instead wished to give Thomas the information he wanted, rather than a lecture that he shouldn’t have had it.
Plus a man raised from the dead is quite a story, and he was late in showing up so yeah you get home and your buddies tell you hey our friend whom we just saw get killed, is right here – you might just think it’s a gag or there is some other disconnect.
Beliefs are something I cannot control. I can wish to believe something, pretend I do, live as if I do, and often I don’t really know whether I believe something. But I can’t just “will” whatever I actually believe in my heart. When it comes to core beliefs, my head can’t fool my heart.
and I would imagine Mary was spared doubts because of her Immaculate Conception, 
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